feat: add server time to obsidian connect responses and enhance error handling

- Included server_time_utc in the connect response schema for better synchronization.
- Updated obsidian_connect function to set server_time_utc during connection handling.
- Enhanced integration tests to verify the presence of server_time_utc in responses.
- Improved connectivity status recovery in the sync engine for better error management.

surfsense_backend/tests/integration/test_obsidian_plugin_routes.py

@@ -343,6 +343,7 @@ class TestWireContractSmoke:
         assert connect_resp.connector_id > 0
         assert connect_resp.vault_id == vault_id
         assert "sync" in connect_resp.capabilities
+        assert connect_resp.server_time_utc is not None
 
         # 2. /sync — stub the indexer so the call doesn't drag the LLM /
         # embedding pipeline in. We're testing the wire contract, not the