refactor(scheduler): drop separate scheduler.db in favour of MemoryJobStore

Scheduler used a separate SQLite file (scheduler.db) only to persist one
cron job. This was originally required because APScheduler's
SQLAlchemyJobStore is sync-only, creating an async/sync driver conflict
when accessing the same file.

The job is rebuilt from config.json on every startup regardless
(replace_existing=True), so the persisted state only served misfire
detection. Moved misfire detection into the app layer by querying
system_settings.last_scan_timestamp on startup: if the last scan is
>23h but <25h ago, an immediate rescan is triggered.

Change summary:
- Remove SQLAlchemyJobStore; use default MemoryJobStore instead
- Add _check_missed_run() that reads last_scan_timestamp from aniworld.db
- Update docs/DEVELOPMENT.md scheduler troubleshooting section
- Update the scheduler unit test that verified SQLAlchemyJobStore

tests/unit/test_scheduler_service.py

@@ -489,12 +489,12 @@ class TestSingletonHelpers:
 
 
 # ---------------------------------------------------------------------------
-# 12.12  Persistent job store — SQLAlchemyJobStore passed to AsyncIOScheduler
+# 12.12  In-memory job store — no separate scheduler.db needed
 # ---------------------------------------------------------------------------
 
-class TestPersistentJobStore:
+class TestInMemoryJobStore:
     @pytest.mark.asyncio
-    async def test_start_creates_scheduler_with_sqlalchemy_jobstore(
+    async def test_start_creates_scheduler_without_jobstore_arg(
         self, scheduler_service, mock_config_service
     ):
         with patch(
@@ -508,10 +508,9 @@ class TestPersistentJobStore:
 
             MockScheduler.assert_called_once()
             call_kwargs = MockScheduler.call_args
-            jobstores = call_kwargs[1]["jobstores"]
-            assert "default" in jobstores
-            # Verify it's a SQLAlchemyJobStore (class check via module name)
-            assert "sqlalchemy" in type(jobstores["default"]).__module__
+            # No jobstores argument — uses default MemoryJobStore
+            if call_kwargs[1]:
+                assert "jobstores" not in call_kwargs[1]
 
     @pytest.mark.asyncio
     async def test_job_options_include_misfire_grace_and_coalesce(