Updater v1 --upgrade + roadmap as flat backlog

Updater: watch-together --upgrade checks updates/manifest.json,
downloads the tarball, verifies sha256, applies over the live
install (extract-temp + delete-first move-over; rename-safe on
Linux while running). Zero Flutter imports, 7 tests incl.
stub-server flows. Version stamped via --dart-define=APP_VERSION
in package script. Warns (never silently jumps) on proto mismatch.
Roadmap: phases retired as tracker (history in git log), now flat
Built/Backlog/Epics/Deferred; Jellyfin next feature, transcode
never; updater expansion stays an epic. Endpoint publishing
(manifest + release tarball) is user-owned.

updates/manifest.example.json

@@ -0,0 +1,7 @@
+{
+  "version": "0.2.0",
+  "protocol": 4,
+  "url": "https://git.senkochan.moe/Foxyz/sync_replay_project/releases/download/v0.2.0/watch-together-0.2.0-linux-x64.tar.gz",
+  "sha256": "<sha256 of the tarball, filled in at release time>",
+  "notes": "One-line highlight of what changed."
+}