Restructure step 6: what's next and watch tracking

- What's next is the server's: state.advance counts down (10 s) when the
  media finishes with an Up next item first, holds while its performer or
  warm source is gone, and sets queuePlay at its end; cancel is shared, a
  seek or swap ends it; the server skips items nobody can put up. Shared
  rules in core whats_next.dart; the app's janitor, per-screen countdown
  and end-of-episode card are gone.
- The continuation (next Jellyfin episode, next file of a local show
  with its gaps) is an auto item under "Continues with", prepared like
  Up next's first item.
- Watch tracking by time In (D3): only time played while this viewer is
  in counts; away, paused, held, jumps and stalls never do (P7, P9).
  WatchTracker + device WatchHistory (files, links); Jellyfin progress
  reads the same watch; records are the AniList seam.
- A server restart no longer loses what played: apps hold their media
  and one offers it back (swap.restore, first offer wins), Up next is
  re-added, the source re-feeds, and the watch carries on.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

apps/desktop/lib/src/watch/watch_history.dart

@@ -0,0 +1,187 @@
+import 'package:flutter/foundation.dart';
+import 'package:hive_ce/hive.dart';
+import 'package:signals/signals_flutter.dart';
+import 'package:sync_relay_core/core.dart';
+
+// The device's watch history (device-owned, one Hive box): what this app
+// watched of files and links, by [WatchKey], with the verdict of the
+// watched rule (`WatchSession`, D3). Jellyfin keeps its own (reported by
+// `JellyfinProgressReporter`); AniList later reads the tracker's records.
+// See ARCHITECTURE.md "Watch tracking".
+
+const watchHistoryBoxName = 'watch_history';
+
+/// What a watch is about, independent of the opening: a file by its path
+/// on this computer, a link by its URL, a Jellyfin item by its ids, else
+/// the media's name and size (someone else's file, a link passed on).
+class WatchKey {
+  final String value;
+  const WatchKey._(this.value);
+
+  factory WatchKey.file(String path) => WatchKey._('file:$path');
+  factory WatchKey.link(String url) => WatchKey._('link:$url');
+  factory WatchKey.jellyfin(JellyfinRef ref) => WatchKey._('jf:${ref.serverId}/${ref.itemId}');
+  factory WatchKey.media(String name, int size) => WatchKey._('media:$name|$size');
+
+  bool get isJellyfin => value.startsWith('jf:');
+
+  /// The key of `state.media` [media]: [ownFile] / [ownLink] when this app
+  /// put it up from there, else its Jellyfin ref, its shared link, its
+  /// name and size.
+  static WatchKey? of(Map<String, Object?>? media, {String? ownFile, String? ownLink}) {
+    if (media == null) return null;
+    if (ownFile != null) return WatchKey.file(ownFile);
+    final jf = media['jellyfin'];
+    if (jf is Map) {
+      final ref = JellyfinRef.fromJson(Map<String, Object?>.from(jf));
+      if (ref.serverId.isNotEmpty && ref.itemId.isNotEmpty) return WatchKey.jellyfin(ref);
+    }
+    if (ownLink != null) return WatchKey.link(ownLink);
+    final url = media['url'];
+    if (url is String && (url.startsWith('http://') || url.startsWith('https://'))) return WatchKey.link(url);
+    final name = media['name'];
+    final size = media['size'];
+    return WatchKey.media(name is String ? name : '', size is num ? size.toInt() : 0);
+  }
+
+  @override
+  bool operator ==(Object other) => other is WatchKey && other.value == value;
+  @override
+  int get hashCode => value.hashCode;
+  @override
+  String toString() => value;
+}
+
+/// One closed watch: what the tracker hands its sinks (the device history,
+/// AniList later).
+class WatchRecord {
+  final WatchKey key;
+  final String title;
+  final MediaRef? ref;
+  final WatchVerdict verdict;
+  /// Seconds the media played while this viewer was In, and its runtime
+  /// (0 when unknown).
+  final double timeIn;
+  final double runtime;
+  final DateTime at;
+  const WatchRecord({
+    required this.key,
+    required this.title,
+    required this.verdict,
+    required this.timeIn,
+    required this.runtime,
+    required this.at,
+    this.ref,
+  });
+
+  @override
+  String toString() => 'WatchRecord($key, $verdict, ${timeIn.round()} s In of ${runtime.round()} s)';
+}
+
+/// What the history holds for a key: watched (played), or where to
+/// resume, and when.
+class WatchEntry {
+  final bool played;
+  /// Seconds; 0 when played or none.
+  final double resumeAt;
+  final String title;
+  final MediaRef? ref;
+  final DateTime at;
+  const WatchEntry({required this.played, this.resumeAt = 0, this.title = '', this.ref, required this.at});
+
+  Map<String, Object?> toJson() => {
+        'played': played,
+        if (resumeAt > 0) 'resumeAt': resumeAt,
+        if (title.isNotEmpty) 'title': title,
+        if (ref != null) 'ref': ref!.toJson(),
+        'at': at.toUtc().toIso8601String(),
+      };
+
+  static WatchEntry? fromJson(Object? j) {
+    if (j is! Map) return null;
+    final at = DateTime.tryParse('${j['at']}');
+    if (at == null) return null;
+    final resume = j['resumeAt'];
+    MediaRef? ref;
+    try {
+      ref = MediaRef.fromJson(j['ref']);
+    } catch (_) {}
+    return WatchEntry(
+      played: j['played'] == true,
+      resumeAt: resume is num && resume.isFinite && resume > 0 ? resume.toDouble() : 0,
+      title: j['title'] is String ? j['title'] as String : '',
+      ref: ref,
+      at: at,
+    );
+  }
+}
+
+/// The device's watch history: [entry] per key, [record] from the
+/// tracker. [revision] bumps on every change (pages that show it listen).
+class WatchHistory {
+  final Map<String, Object?> _memory = {};
+  /// False: memory only (tests).
+  final bool persist;
+  Box? _box;
+  Future<void>? _load;
+  final revision = signal(0);
+
+  WatchHistory({this.persist = true});
+
+  Future<void> ensureLoaded() => _load ??= () async {
+        if (!persist) return;
+        try {
+          final box = _box = await Hive.openBox(watchHistoryBoxName);
+          for (final e in _memory.entries) {
+            await box.put(e.key, e.value);
+          }
+          _memory.clear();
+          revision.value++;
+        } catch (e, st) {
+          debugPrint('open watch history failed: $e\n$st');
+        }
+      }();
+
+  WatchEntry? entry(WatchKey key) => WatchEntry.fromJson(_box?.get(key.value) ?? _memory[key.value]);
+
+  /// A closed watch: played -> played (the resume point goes); part way
+  /// -> its resume point, unless it was played before (it stays played);
+  /// a peek changes nothing. Jellyfin items are not kept here.
+  void record(WatchRecord r) {
+    if (r.key.isJellyfin) return;
+    final before = entry(r.key);
+    final WatchEntry next;
+    switch (r.verdict.outcome) {
+      case WatchOutcome.played || WatchOutcome.keepPlayed:
+        next = WatchEntry(played: true, title: r.title, ref: r.ref, at: r.at);
+      case WatchOutcome.partial:
+        next = WatchEntry(
+            played: before?.played ?? false,
+            resumeAt: (before?.played ?? false) ? 0 : r.verdict.resumeAt ?? 0,
+            title: r.title,
+            ref: r.ref,
+            at: r.at);
+      case WatchOutcome.peek:
+        return;
+    }
+    final json = next.toJson();
+    final box = _box;
+    if (box == null) {
+      _memory[r.key.value] = json;
+    } else {
+      box.put(r.key.value, json).catchError((Object e) => debugPrint('watch history put ${r.key}: $e'));
+    }
+    revision.value++;
+  }
+
+  /// Every entry, newest first.
+  List<(WatchKey, WatchEntry)> get entries {
+    final raw = <String, Object?>{..._memory, if (_box != null) for (final k in _box!.keys) '$k': _box!.get(k)};
+    final out = [
+      for (final e in raw.entries)
+        if (WatchEntry.fromJson(e.value) case final w?) (WatchKey._(e.key), w),
+    ];
+    out.sort((a, b) => b.$2.at.compareTo(a.$2.at));
+    return out;
+  }
+}

apps/desktop/lib/src/watch/watch_tracker.dart

@@ -0,0 +1,159 @@
+import 'dart:async';
+
+import 'package:sync_relay_core/core.dart';
+
+import 'watch_history.dart';
+
+// Watch tracking (ARCHITECTURE.md "Watch tracking", D3): one watch per
+// media of the server (its `mediaId`), fed every second by the session
+// with what this app sees, judged by core's `WatchSession` when it ends.
+// The verdict goes to the device history (files and links) and out on
+// [WatchTracker.records] (the seam for AniList later); Jellyfin's
+// played/resume reports read the same session.
+
+final _log = createLogger('watch');
+
+/// What the session hands the tracker each second.
+class WatchSample {
+  /// The server's media and its id.
+  final int mediaId;
+  final Map<String, Object?>? media;
+  /// This app's own source of it: the file it put up, the link it put up.
+  final String? ownFile;
+  final String? ownLink;
+  final double position;
+  /// The media plays here: the server plays it and this player has it.
+  final bool playing;
+  /// This viewer is In (`in`): not away, joining or gone.
+  final bool present;
+  /// The player's duration (0 while unknown) and where the ending starts.
+  final double duration;
+  final double? endingStart;
+  final DateTime at;
+  const WatchSample({
+    required this.mediaId,
+    required this.media,
+    required this.position,
+    required this.playing,
+    required this.present,
+    required this.at,
+    this.ownFile,
+    this.ownLink,
+    this.duration = 0,
+    this.endingStart,
+  });
+}
+
+class WatchTracker {
+  final WatchHistory history;
+  WatchTracker({WatchHistory? history}) : history = history ?? WatchHistory(persist: false);
+
+  final _records = StreamController<WatchRecord>.broadcast();
+
+  /// Every closed watch (peeks too): the device history takes files and
+  /// links; AniList progress later listens here.
+  Stream<WatchRecord> get records => _records.stream;
+
+  int _mediaId = -1;
+  WatchKey? _key;
+  String _title = '';
+  MediaRef? _ref;
+  double _runtime = 0;
+  double? _endingStart;
+  WatchSession? _session;
+
+  /// The watch of the media playing (null before the first sample).
+  WatchSession? get session => _session;
+  WatchKey? get key => _key;
+
+  /// One second of the session. A new media closes the watch of the one
+  /// before.
+  void sample(WatchSample s) {
+    if (s.mediaId != _mediaId) {
+      close(at: s.at);
+      if (s.media == null) return;
+      _open(s);
+    }
+    final session = _session;
+    if (session == null) return;
+    // A file or link this app put up is known once its swap landed.
+    final key = WatchKey.of(s.media, ownFile: s.ownFile, ownLink: s.ownLink);
+    if (key != null && key != _key && !(_key?.isJellyfin ?? false)) _key = key;
+    if (s.duration > 0) _runtime = s.duration;
+    _endingStart = s.endingStart;
+    session.sample(s.position, playing: s.playing, present: s.present, at: s.at);
+  }
+
+  void _open(WatchSample s) {
+    final key = WatchKey.of(s.media, ownFile: s.ownFile, ownLink: s.ownLink);
+    final before = key == null ? null : history.entry(key);
+    _mediaId = s.mediaId;
+    _key = key;
+    _title = '${s.media?['name'] ?? ''}';
+    try {
+      _ref = MediaRef.fromJson(s.media?['ref']);
+    } catch (_) {
+      _ref = null;
+    }
+    final d = s.media?['duration'];
+    _runtime = d is num && d > 0 ? d.toDouble() : 0;
+    _endingStart = null;
+    _session = WatchSession(playedBefore: before?.played ?? false, resumeBefore: before?.resumeAt ?? 0);
+  }
+
+  /// The same media goes on under another [mediaId] (a restarted server
+  /// numbers it anew): the watch carries on.
+  void renumber(int from, int to) {
+    if (_mediaId == from) _mediaId = to;
+  }
+
+  /// What Jellyfin says about the item before this opening (its played
+  /// state and resume point, read once it is open).
+  void before({required bool played, double resumeAt = 0}) {
+    final session = _session;
+    if (session == null) return;
+    session
+      ..playedBefore = played
+      ..resumeBefore = resumeAt;
+  }
+
+  /// The verdict so far (the runtime as last seen); null without a watch.
+  WatchVerdict? get verdict => _session?.verdict(runtime: _runtime, endingStart: _endingStart);
+
+  /// Records the watch as it stands (the end of the media; it may go on
+  /// after a seek back and be recorded again).
+  WatchRecord? checkpoint({DateTime? at}) {
+    final session = _session, key = _key;
+    if (session == null || key == null) return null;
+    final v = session.verdict(runtime: _runtime, endingStart: _endingStart);
+    final r = WatchRecord(
+        key: key,
+        title: _title,
+        ref: _ref,
+        verdict: v,
+        timeIn: session.timeIn,
+        runtime: _runtime,
+        at: at ?? DateTime.now().toUtc());
+    _log.info('$key: ${v.outcome.name} (${session.timeIn.round()} s In of ${_runtime.round()} s)');
+    history.record(r);
+    _records.add(r);
+    return r;
+  }
+
+  /// The watch ends (another media, leaving): recorded and forgotten.
+  WatchRecord? close({DateTime? at}) {
+    final r = _session == null ? null : checkpoint(at: at);
+    _mediaId = -1;
+    _key = null;
+    _session = null;
+    _ref = null;
+    _runtime = 0;
+    _endingStart = null;
+    return r;
+  }
+
+  /// The media the current watch is of (-1: none).
+  int get mediaId => _mediaId;
+
+  Future<void> dispose() => _records.close();
+}