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/models/translate.py

@@ -239,6 +239,7 @@ class TranslationSchedule(Base):
     is_active = Column(Boolean, default=True)
     last_run_at = Column(DateTime, nullable=True)
     next_run_at = Column(DateTime, nullable=True)
+    execution_mode = Column(String, nullable=False, default="full", comment="full, new_key_only")
     created_by = Column(String, nullable=True)
     created_at = Column(DateTime, default=lambda: datetime.now(timezone.utc))
     updated_at = Column(DateTime, default=lambda: datetime.now(timezone.utc), onupdate=lambda: datetime.now(timezone.utc))