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/services/clean_release/test_demo_mode_isolation.py

@@ -1,5 +1,4 @@
 # [DEF:TestDemoModeIsolation:Module]
-# @COMPLEXITY: 3
 # @SEMANTICS: clean-release, demo-mode, isolation, namespace, repository
 # @PURPOSE: Verify demo and real mode namespace isolation contracts before TUI integration.
 # @LAYER: Tests