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/tests/conftest.py

@@ -0,0 +1,25 @@
+# #region TestSessionConfig [C:2] [TYPE Module] [SEMANTICS test, conftest, db]
+# @BRIEF Shared pytest fixtures and session configuration for backend tests.
+# @RELATION BINDS_TO -> [init_db]
+# @PRE All test modules use sys.path patching to import from src/.
+# @POST Database tables exist before test session executes.
+import sys
+from pathlib import Path
+
+sys.path.insert(0, str(Path(__file__).parent.parent / "src"))
+
+import pytest
+
+
+@pytest.fixture(scope="session", autouse=True)
+def ensure_db_tables():
+    """Ensure all tables exist before test session.
+
+    This fixture runs once per test session, creating all registered
+    SQLAlchemy tables. Individual test modules may also create their
+    own in-memory engines — this guarantees the real engine schema
+    is initialized for tests that depend on it.
+    """
+    from src.core.database import init_db
+    init_db()
+# #endregion TestSessionConfig