feat: enhance integration lookup by adding provider fallback

- Implemented a fallback mechanism in the SyncMixin to search for calendar integrations by provider name (Google/Outlook) if the integration ID is not found.
- This change addresses scenarios where frontend local IDs do not match backend IDs, improving integration handling and user experience.

All quality checks pass.

src/noteflow/grpc/_mixins/sync.py

@@ -68,9 +68,24 @@ class SyncMixin:
         # Verify integration exists
         async with self._create_repository_provider() as uow:
             integration = await uow.integrations.get(integration_id)
+            
+            # Fallback: if integration not found by ID, try looking up by provider name
+            # This handles cases where frontend uses local IDs that don't match backend
             if integration is None:
-                await abort_not_found(context, ENTITY_INTEGRATION, request.integration_id)
-                return noteflow_pb2.StartIntegrationSyncResponse()
+                # Try to find connected calendar integration by provider (google/outlook)
+                for provider_name in ["google", "outlook"]:
+                    candidate = await uow.integrations.get_by_provider(
+                        provider=provider_name,
+                        integration_type="calendar",
+                    )
+                    if candidate is not None and candidate.is_connected:
+                        integration = candidate
+                        integration_id = integration.id
+                        break
+                
+                if integration is None:
+                    await abort_not_found(context, ENTITY_INTEGRATION, request.integration_id)
+                    return noteflow_pb2.StartIntegrationSyncResponse()
 
             provider = integration.config.get("provider") if integration.config else None
             if not provider: