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/core/test_mapping_service.py

@@ -1,6 +1,5 @@
 # [DEF:TestMappingService:Module]
 #
-# @COMPLEXITY: 3
 # @PURPOSE:   Unit tests for the IdMappingService matching UUIDs to integer IDs.
 # @LAYER:     Domain
 # @RELATION:  VERIFIES ->[src.core.mapping_service.IdMappingService]