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/spaces/data/spaces_repository.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
 // SpacesRepository — builds space list and child rooms from the Matrix SDK.
 // A space is a room where isSpace == true.
 
@@ -6,6 +6,7 @@ import 'package:matrix/matrix.dart';
 import 'package:riverpod_annotation/riverpod_annotation.dart';
 
 import '../../../core/network/matrix_client.dart';
+import '../../../shared/utils/mxc_url.dart';
 import '../domain/space_model.dart';
 
 part 'spaces_repository.g.dart';
@@ -46,7 +47,7 @@ class SpacesRepository {
         SpaceRoomModel(
           id: room.id,
           displayName: room.getLocalizedDisplayname(),
-          avatarUrl: room.avatar?.toString(),
+          avatarUrl: resolveMxcUrl(_client, room.avatar),
           isDirect: room.isDirectChat,
         ),
       );
@@ -64,7 +65,7 @@ class SpacesRepository {
     return SpaceModel(
       id: room.id,
       displayName: room.getLocalizedDisplayname(),
-      avatarUrl: room.avatar?.toString(),
+      avatarUrl: resolveMxcUrl(_client, room.avatar),
       roomCount: room.spaceChildren.length,
     );
   }