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/api/routes/translate/_schedule_routes.py

@@ -44,6 +44,7 @@ async def get_schedule(
             "cron_expression": schedule.cron_expression,
             "timezone": schedule.timezone,
             "is_active": schedule.is_active,
+            "execution_mode": schedule.execution_mode,
             "last_run_at": schedule.last_run_at.isoformat() if schedule.last_run_at else None,
             "next_run_at": schedule.next_run_at.isoformat() if schedule.next_run_at else None,
             "created_by": schedule.created_by,
@@ -77,12 +78,14 @@ async def set_schedule(
         existing = db.query(TranslationSchedule).filter(
             TranslationSchedule.job_id == job_id
         ).first()
+        execution_mode = getattr(payload, 'execution_mode', 'full')
         if existing:
             schedule = scheduler.update_schedule(
                 job_id,
                 cron_expression=payload.cron_expression,
                 timezone_str=payload.timezone,
                 is_active=payload.is_active,
+                execution_mode=execution_mode,
             )
         else:
             schedule = scheduler.create_schedule(
@@ -90,6 +93,7 @@ async def set_schedule(
                 cron_expression=payload.cron_expression,
                 timezone=payload.timezone,
                 is_active=payload.is_active,
+                execution_mode=execution_mode,
             )
         # Register with APScheduler via SchedulerService
         from ....dependencies import get_scheduler_service
@@ -99,6 +103,7 @@ async def set_schedule(
             job_id=job_id,
             cron_expression=schedule.cron_expression,
             timezone=schedule.timezone,
+            execution_mode=schedule.execution_mode,
         )
         return {
             "id": schedule.id,
@@ -106,6 +111,7 @@ async def set_schedule(
             "cron_expression": schedule.cron_expression,
             "timezone": schedule.timezone,
             "is_active": schedule.is_active,
+            "execution_mode": schedule.execution_mode,
             "last_run_at": schedule.last_run_at.isoformat() if schedule.last_run_at else None,
             "created_by": schedule.created_by,
             "created_at": schedule.created_at.isoformat() if schedule.created_at else None,
@@ -140,6 +146,7 @@ async def enable_schedule(
             job_id=job_id,
             cron_expression=schedule.cron_expression,
             timezone=schedule.timezone,
+            execution_mode=schedule.execution_mode,
         )
         return {"status": "enabled", "job_id": job_id}
     except ValueError as e: