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/application/ports.dart

@@ -22,6 +22,24 @@ abstract interface class AuthSessionRepository {
   Future<void> revoke({required String sessionId, required DateTime revokedAt});
 }
 
+enum SyncWriteStatus { accepted, ignoredOlder }
+
+final class SyncWriteResult {
+  const SyncWriteResult({required this.status, required this.resource});
+
+  final SyncWriteStatus status;
+  final SyncedResource resource;
+}
+
+abstract interface class SyncedResourceRepository {
+  Future<SyncWriteResult> upsertWithLww(SyncedResource resource);
+
+  Future<List<SyncedResource>> findAllForUserSince({
+    required String ownerUserId,
+    DateTime? since,
+  });
+}
+
 abstract interface class PasswordHasher {
   Future<String> hash(String password);