feat(server): API de synchronisation incrémentale LWW (ticket #50)

Ajoute l'endpoint api/sync_api.dart, les use cases de synchronisation
(application/sync_use_cases.dart) et l'adapter Postgres
(infrastructure/postgres/synced_resource_repository.dart) implémentant
un upsert LWW atomique via CTE (INSERT ... ON CONFLICT ... WHERE
client_updated_at < EXCLUDED.client_updated_at, avec fallback UNION
ALL pour le cas ignoré). dart pub get OK, dart analyze clean, dart
test 15/15 vert. SQL d'upsert relu manuellement et jugé correct ; pas
de test de bout en bout contre un vrai PostgreSQL faute d'accès
Docker dans ce sandbox.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/lib/domain/entities.dart

@@ -88,6 +88,63 @@ final class AuthSession {
   }
 }
 
+enum SyncedResourceType {
+  exercise('exercise'),
+  program('program'),
+  workoutTemplate('workoutTemplate'),
+  workoutHistory('workoutHistory'),
+  mediaAsset('mediaAsset');
+
+  const SyncedResourceType(this.wireName);
+
+  final String wireName;
+
+  static SyncedResourceType parse(String value) {
+    for (final type in values) {
+      if (type.wireName == value) {
+        return type;
+      }
+    }
+    throw ValidationException('Unsupported resource type: $value.');
+  }
+}
+
+final class SyncedResource {
+  SyncedResource({
+    required String serverId,
+    required String ownerUserId,
+    required this.resourceType,
+    required String clientId,
+    required Map<String, Object?> payloadJson,
+    required this.schemaVersion,
+    required DateTime clientUpdatedAt,
+    required DateTime serverUpdatedAt,
+    DateTime? deletedAt,
+    this.originDeviceId,
+  }) : serverId = _nonBlank(serverId, 'Server resource id'),
+       ownerUserId = _nonBlank(ownerUserId, 'Owner user id'),
+       clientId = _nonBlank(clientId, 'Client resource id'),
+       payloadJson = Map.unmodifiable(payloadJson),
+       clientUpdatedAt = clientUpdatedAt.toUtc(),
+       serverUpdatedAt = serverUpdatedAt.toUtc(),
+       deletedAt = deletedAt?.toUtc() {
+    if (schemaVersion <= 0) {
+      throw const ValidationException('Schema version must be positive.');
+    }
+  }
+
+  final String serverId;
+  final String ownerUserId;
+  final SyncedResourceType resourceType;
+  final String clientId;
+  final Map<String, Object?> payloadJson;
+  final int schemaVersion;
+  final DateTime clientUpdatedAt;
+  final DateTime serverUpdatedAt;
+  final DateTime? deletedAt;
+  final String? originDeviceId;
+}
+
 String _nonBlank(String value, String label) {
   final trimmed = value.trim();
   if (trimmed.isEmpty) {