feat: Initial MVP with home server identity model

- User model with Ed25519 keypair identity
- Register/login on home server (creates keypair)
- Join guest servers with signature verification
- Public endpoints for cross-server identity
- Channel and message CRUD
- JWT authentication middleware

Architecture: Home server model, no federation, client-driven sync

lib/models.dart

@@ -0,0 +1,126 @@
+import 'package:objectbox/objectbox.dart';
+
+/// User entity - works for both HOME and GUEST servers
+/// 
+/// On HOME server:
+/// - isLocal = true
+/// - stores password, encrypted private key
+/// - this is where identity lives
+/// 
+/// On GUEST server:
+/// - isLocal = false
+/// - homeServer contains the URL of user's home
+/// - publicKey cached for verification
+@Entity()
+class User {
+  @Id()
+  int id = 0;
+
+  // Global identifier: "username@homeserver.com"
+  @Unique()
+  String userId;
+
+  // Basic profile
+  @Index()
+  String username;
+  String? displayName;
+  String? avatarUrl;
+  String? bio;
+
+  // Identity
+  String publicKey; // Ed25519 public key, always stored
+
+  // HOME SERVER ONLY: Auth & key backup
+  String? passwordHash;
+  String? encryptedPrivateKey; // Encrypted with user's password
+  String? keyPairSalt;
+
+  // Server relationship
+  bool isLocal; // true = this is their home server
+  String? homeServer; // if guest, where is their home? (e.g., "https://my.server.com")
+
+  // Server-specific
+  List<String> roles; // ["admin", "moderator", "member"]
+
+  @Property(type: PropertyType.date)
+  DateTime createdAt;
+
+  @Property(type: PropertyType.date)
+  DateTime? lastSyncedAt; // When profile was last refreshed from home
+
+  User({
+    required this.userId,
+    required this.username,
+    required this.publicKey,
+    required this.isLocal,
+    this.displayName,
+    this.avatarUrl,
+    this.bio,
+    this.passwordHash,
+    this.encryptedPrivateKey,
+    this.keyPairSalt,
+    this.homeServer,
+    List<String>? roles,
+    DateTime? createdAt,
+    DateTime? lastSyncedAt,
+  })  : roles = roles ?? ['member'],
+        createdAt = createdAt ?? DateTime.now(),
+        lastSyncedAt = lastSyncedAt;
+}
+
+@Entity()
+class Channel {
+  @Id()
+  int id = 0;
+
+  @Unique()
+  String name;
+
+  String? description;
+
+  @Property(type: PropertyType.date)
+  DateTime createdAt;
+
+  Channel({
+    required this.name,
+    this.description,
+    DateTime? createdAt,
+  }) : createdAt = createdAt ?? DateTime.now();
+}
+
+@Entity()
+class Message {
+  @Id()
+  int id = 0;
+
+  int channelId;
+
+  // Author info - includes enough for cross-server profile fetch
+  int authorLocalId;
+  String authorUserId; // "alice@homeserver.com"
+  String? authorHomeServer; // for profile fetching
+
+  // Cached for display (snapshot at message time)
+  String authorUsername;
+  String? authorAvatarUrl;
+
+  String content;
+
+  @Property(type: PropertyType.date)
+  DateTime createdAt;
+
+  @Property(type: PropertyType.date)
+  DateTime? editedAt;
+
+  Message({
+    required this.channelId,
+    required this.authorLocalId,
+    required this.authorUserId,
+    required this.authorUsername,
+    required this.content,
+    this.authorHomeServer,
+    this.authorAvatarUrl,
+    DateTime? createdAt,
+    this.editedAt,
+  }) : createdAt = createdAt ?? DateTime.now();
+}