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/core/database.py

@@ -579,6 +579,96 @@ def _ensure_dataset_review_session_columns(bind_engine):
 # #endregion _ensure_dataset_review_session_columns
 
 
+# #region _ensure_translation_schedules_columns [C:3] [TYPE Function]
+# @BRIEF Applies additive schema upgrades for translation_schedules table.
+# @PRE: bind_engine points to application database.
+# @POST: Missing columns are added without data loss.
+# @RELATION DEPENDS_ON -> [engine]
+def _ensure_translation_schedules_columns(bind_engine):
+    with belief_scope("_ensure_translation_schedules_columns"):
+        table_name = "translation_schedules"
+        inspector = inspect(bind_engine)
+        if table_name not in inspector.get_table_names():
+            return
+
+        existing_columns = {
+            str(column.get("name") or "").strip()
+            for column in inspector.get_columns(table_name)
+        }
+
+        alter_statements = []
+        if "execution_mode" not in existing_columns:
+            alter_statements.append(
+                "ALTER TABLE translation_schedules "
+                "ADD COLUMN execution_mode VARCHAR NOT NULL DEFAULT 'full'"
+            )
+
+        if not alter_statements:
+            return
+
+        try:
+            with bind_engine.begin() as connection:
+                for statement in alter_statements:
+                    connection.execute(text(statement))
+        except Exception as migration_error:
+            logger.explore(
+                "TranslationSchedule additive migration failed",
+                extra={"src": "database", "error": str(migration_error)},
+            )
+
+
+# #endregion _ensure_translation_schedules_columns
+
+
+# #region _ensure_dictionary_entries_columns [C:3] [TYPE Function]
+# @BRIEF Additive migration for dictionary_entries origin tracking columns.
+# @RELATION DEPENDS_ON -> [engine]
+def _ensure_dictionary_entries_columns(bind_engine):
+    with belief_scope("_ensure_dictionary_entries_columns"):
+        table_name = "dictionary_entries"
+        inspector = inspect(bind_engine)
+        if table_name not in inspector.get_table_names():
+            return
+
+        existing_columns = {
+            str(column.get("name") or "").strip()
+            for column in inspector.get_columns(table_name)
+        }
+
+        alter_statements = []
+        if "origin_run_id" not in existing_columns:
+            alter_statements.append(
+                "ALTER TABLE dictionary_entries "
+                "ADD COLUMN origin_run_id VARCHAR"
+            )
+        if "origin_row_key" not in existing_columns:
+            alter_statements.append(
+                "ALTER TABLE dictionary_entries "
+                "ADD COLUMN origin_row_key VARCHAR"
+            )
+        if "origin_user_id" not in existing_columns:
+            alter_statements.append(
+                "ALTER TABLE dictionary_entries "
+                "ADD COLUMN origin_user_id VARCHAR"
+            )
+
+        if not alter_statements:
+            return
+
+        try:
+            with bind_engine.begin() as connection:
+                for statement in alter_statements:
+                    connection.execute(text(statement))
+        except Exception as migration_error:
+            logger.explore(
+                "DictionaryEntry additive migration failed",
+                extra={"src": "database", "error": str(migration_error)},
+            )
+
+
+# #endregion _ensure_dictionary_entries_columns
+
+
 # #region init_db [C:3] [TYPE Function]
 # @BRIEF Initializes the database by creating all tables.
 # @PRE: engine, tasks_engine and auth_engine are initialized.
@@ -587,6 +677,7 @@ def _ensure_dataset_review_session_columns(bind_engine):
 # @RELATION CALLS -> [ensure_connection_configs_table]
 # @RELATION CALLS -> [_ensure_filter_source_enum_values]
 # @RELATION CALLS -> [_ensure_dataset_review_session_columns]
+# @RELATION CALLS -> [_ensure_dictionary_entries_columns]
 def init_db():
     with belief_scope("init_db"):
         Base.metadata.create_all(bind=engine)
@@ -601,6 +692,8 @@ def init_db():
         _ensure_filter_source_enum_values(engine)
         _ensure_dataset_review_session_columns(engine)
         _ensure_translation_jobs_columns(engine)
+        _ensure_translation_schedules_columns(engine)
+        _ensure_dictionary_entries_columns(engine)
 
 
 # #endregion init_db