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_migration_engine.py

@@ -1,6 +1,5 @@
 # [DEF:TestMigrationEngine:Module]
 #
-# @COMPLEXITY: 3
 # @PURPOSE:   Unit tests for MigrationEngine's cross-filter patching algorithms.
 # @LAYER:     Domain
 # @RELATION: VERIFIES -> [src.core.migration_engine:Module]
@@ -29,7 +28,6 @@ from src.models.mapping import ResourceType
 
 # [DEF:MockMappingService:Class]
 # @RELATION: BINDS_TO -> [TestMigrationEngine:Module]
-# @COMPLEXITY: 2
 # @PURPOSE: Deterministic mapping service double for native filter ID remapping scenarios.
 # @INVARIANT: Returns mappings only for requested UUID keys present in seeded map.
 class MockMappingService: