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/features/calls/presentation/call_controller.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
 // Call controller — manages LiveKit room connection lifecycle.
 // Transitions through idle → connecting → active → ended states.
 
@@ -12,13 +12,20 @@ import '../domain/call_state.dart';
 
 part 'call_controller.g.dart';
 
-@Riverpod(keepAlive: false)
+@Riverpod(keepAlive: true)
 class CallController extends _$CallController {
   Timer? _durationTimer;
   Duration _elapsed = Duration.zero;
 
   @override
-  CallState build() => const CallState.idle();
+  CallState build() {
+    ref.onDispose(() {
+      _durationTimer?.cancel();
+      _durationTimer = null;
+      ref.read(liveKitServiceProvider).disconnect();
+    });
+    return const CallState.idle();
+  }
 
   /// Join a LiveKit room via MatrixRTC JWT endpoint.
   ///