fix(llm): add fetch-models endpoint, fix SQL Lab INSERT (client_id truncation, sync mode, target_column, timestamp normalization)

- Add POST /api/llm/providers/fetch-models route with LLMClient.fetch_models()
- Add target_column to TranslationJob model/schema/service/orchestrator
- Fix SQL Lab execute: truncate client_id to 11 chars (varchar(11))
- Switch SQL Lab to sync mode (runAsync: false) — no Celery workers
- Fix polling: unwrap nested result from Superset query API
- Fix ClickHouse timestamp: normalize float timestamps to YYYY-MM-DD

backend/tests/test_resource_hubs.py

@@ -1,7 +1,6 @@
 # [DEF:TestResourceHubs:Module]
 # @RELATION: DEPENDS_ON -> [DashboardsApi]
 # @RELATION: DEPENDS_ON -> [DatasetsApi]
-# @COMPLEXITY: 3
 # @SEMANTICS: tests, resource-hubs, dashboards, datasets, pagination, api
 # @PURPOSE: Contract tests for resource hub dashboards/datasets listing and pagination boundary validation.
 # @LAYER: Domain (Tests)