refactor: /simplify — 22 fixes from 3-agent code review

Critical:
- Fix MXC URI resolution: all avatars/images now resolve mxc:// to HTTP
- Sync persistence: only write changed rooms, batch message upserts
- lastActivityAt uses room.lastEvent.originServerTs, not creation time

High:
- Shared MatrixAvatar widget replaces 6 duplicate implementations
- CallScreen decodes roomId before LiveKit JWT fetch
- Decline button actually dismisses incoming call overlay
- EventTypes constants replace raw string literals
- LiveKitService uses lazy auth reads, onDispose disconnects

Medium:
- CallController is keepAlive with timer/room cleanup
- authRepository is keepAlive (used from keepAlive notifier)
- StreamController not closed in stopListening (crash fix)
- Index on messages.roomId for query performance
- 400ms debounce on user search
- Static DateFormat in MessageBubble
- Hardcoded strings replaced with AppConfig refs
- Duplicate isDirectMessage field removed from RoomModel
- E2EE profile claim corrected to Phase 3

Shared utilities:
- lib/shared/widgets/matrix_avatar.dart
- lib/shared/utils/mxc_url.dart
- lib/shared/utils/room_preview.dart
- lib/shared/utils/matrix_id.dart

rawJson column removed (unused, caused main-thread jsonEncode)
Schema migrated to v2 with roomId index.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

lib/core/storage/database.dart

@@ -1,4 +1,4 @@
-// Version: 1.1.0 | Created: 2026-04-01
+// Version: 1.2.0 | Created: 2026-04-01 | Updated: 2026-04-02
 // Drift database — rooms and messages tables.
 // Uses driftDatabase() from drift_flutter for cross-platform support:
 //   - Mobile/desktop: SQLite file via sqlite3_flutter_libs
@@ -33,6 +33,7 @@ class RoomsTable extends Table {
 }
 
 /// Persisted message events. Upserted when received from sync.
+@TableIndex(name: 'idx_messages_room_id', columns: {#roomId})
 class MessagesTable extends Table {
   @override
   String get tableName => 'messages';
@@ -43,7 +44,6 @@ class MessagesTable extends Table {
   TextColumn get body => text().nullable()();
   TextColumn get type => text()(); // MessageType name
   IntColumn get timestamp => integer()(); // milliseconds since epoch
-  TextColumn get rawJson => text()(); // full event JSON for future use
 
   @override
   Set<Column<Object>> get primaryKey => {id};
@@ -61,7 +61,41 @@ class AppDatabase extends _$AppDatabase {
   AppDatabase.forTesting(super.executor);
 
   @override
-  int get schemaVersion => 1;
+  int get schemaVersion => 2;
+
+  @override
+  MigrationStrategy get migration => MigrationStrategy(
+    onUpgrade: (migrator, from, to) async {
+      if (from < 2) {
+        // v2: drop rawJson column, add index on roomId.
+        // Drift cannot drop columns — recreate the table.
+        await customStatement(
+          'CREATE TABLE IF NOT EXISTS messages_new ('
+          '  id TEXT NOT NULL PRIMARY KEY, '
+          '  room_id TEXT NOT NULL, '
+          '  sender_id TEXT NOT NULL, '
+          '  body TEXT, '
+          '  type TEXT NOT NULL, '
+          '  timestamp INTEGER NOT NULL'
+          ')',
+        );
+        await customStatement(
+          'INSERT OR IGNORE INTO messages_new '
+          '(id, room_id, sender_id, body, type, timestamp) '
+          'SELECT id, room_id, sender_id, body, type, timestamp FROM messages',
+        );
+        await customStatement('DROP TABLE IF EXISTS messages');
+        await customStatement('ALTER TABLE messages_new RENAME TO messages');
+        await customStatement(
+          'CREATE INDEX IF NOT EXISTS idx_messages_room_id '
+          'ON messages (room_id)',
+        );
+      }
+    },
+    onCreate: (migrator) async {
+      await migrator.createAll();
+    },
+  );
 }
 
 // ---------------------------------------------------------------------------
@@ -99,6 +133,15 @@ extension MessagesDao on AppDatabase {
   Future<void> upsertMessage(MessagesTableCompanion message) =>
       into(messagesTable).insertOnConflictUpdate(message);
 
+  /// Insert or replace multiple message rows in a single batch transaction.
+  Future<void> upsertMessages(List<MessagesTableCompanion> messages) async {
+    await batch((b) {
+      for (final msg in messages) {
+        b.insert(messagesTable, msg, onConflict: DoUpdate((_) => msg));
+      }
+    });
+  }
+
   /// Watch all messages for [roomId] ordered oldest-first.
   Stream<List<MessagesTableData>> watchByRoom(String roomId) {
     return (select(messagesTable)