feat(translate,scheduler,trace): new-key-only mode, stale PENDING protection, trace_id propagation

Translations:
- FR-045: new-key-only execution mode — translate only rows with unseen keys
- Compare source row keys against TranslationRecord.source_data from last succeeded run
- Baseline expired (>90 days) fallback to full mode with baseline_expired event
- run_noop early return when zero new keys (skip LLM + SQL)

Scheduler reliability:
- Stale PENDING protection: runs older than 1h auto-marked FAILED, no longer block schedule
- load_schedules() reloads active translation schedules from DB on restart
- add_translation_job/remove_translation_job register/unregister with APScheduler
- execution_mode column on translation_schedules with additive DB migration

Automation view:
- GET /settings/automation/translation-schedules endpoint
- Translation schedules displayed on Automation page below validation policies

Trace propagation:
- seed_trace_id() in all background entry points:
  TaskManager._run_task/_flusher_loop, SchedulerService._trigger_backup,
  websocket_endpoint, IdMappingService, all standalone scripts

Migrations:
- dictionary_entries: origin_run_id, origin_row_key, origin_user_id
- translation_schedules: execution_mode

Tests:
- 3 new test modules (22 tests): core_scheduler, executor_filter, scheduler_execution+guard
- Fix pre-existing translate test isolation (conftest.py)
- Fix test_list_runs_filter_status parameter

backend/src/scripts/test_dataset_dashboard_relations.py

@@ -18,10 +18,12 @@ sys.path.append(str(Path(__file__).parent.parent.parent))
 from src.core.superset_client import SupersetClient
 from src.core.config_manager import ConfigManager
 from src.core.logger import logger
+from src.core.cot_logger import seed_trace_id
 
 
 def test_dashboard_dataset_relations():
     """Test fetching dataset-to-dashboard relationships."""
+    seed_trace_id()
     
     # Load environment from existing config
     config_manager = ConfigManager()