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/src/services/git_service.py

@@ -1,5 +1,4 @@
 # #region git_service [C:1] [TYPE Module:Tombstone] [SEMANTICS git, service, shim, re-export, decomissioned]
-# @COMPLEXITY: 1
 # @BRIEF Re-export shim — GitService has been decomposed into services/git/ package.
 #           All consumers continue to import from this same path without changes.
 # @RELATION REDIRECTS_TO -> [GitServiceModule]