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

src/server/services/scheduler_service.py

@@ -4,17 +4,16 @@ Uses APScheduler's AsyncIOScheduler with CronTrigger for precise
 cron-based scheduling.  The legacy interval-based loop has been removed
 in favour of the cron approach.
 
-Jobs are persisted to a SQLite database so they survive process restarts.
-On startup, if the last scheduled run was missed (server was down at the
-cron time), the job is triggered immediately within a grace period.
+Jobs are held in memory (no separate scheduler database).  On startup,
+if the last scan timestamp indicates a missed run (server was down at the
+scheduled cron time), a rescan is triggered immediately.
 """
 from __future__ import annotations
 
 import logging
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 from typing import List, Optional
 
-from apscheduler.jobstores.sqlalchemy import SQLAlchemyJobStore
 from apscheduler.schedulers.asyncio import AsyncIOScheduler
 from apscheduler.triggers.cron import CronTrigger
 
@@ -83,10 +82,9 @@ class SchedulerService:
             logger.error("Failed to load scheduler configuration: %s", exc)
             raise SchedulerServiceError(f"Failed to load config: {exc}") from exc
 
-        jobstores = {
-            "default": SQLAlchemyJobStore(url="sqlite:///./data/scheduler.db"),
-        }
-        self._scheduler = AsyncIOScheduler(jobstores=jobstores)
+        # Use in-memory job store — no separate scheduler.db needed.
+        # Jobs are reconstructed from config on every startup.
+        self._scheduler = AsyncIOScheduler()
 
         if not self._config.enabled:
             logger.info("Scheduler is disabled in configuration — not adding jobs")
@@ -125,10 +123,7 @@ class SchedulerService:
         self._scheduler.start()
         self._is_running = True
 
-        # Startup recovery: if the server was down at the scheduled time and
-        # the job is within the misfire window, APScheduler will run it
-        # automatically.  Log the scheduled time for visibility.
-        # Note: next_run_time is only available AFTER scheduler.start()
+        # Log next scheduled run for visibility.
         job = self._scheduler.get_job(_JOB_ID)
         if job:
             next_run = job.next_run_time
@@ -137,6 +132,11 @@ class SchedulerService:
                 next_run.isoformat() if next_run else None,
             )
 
+        # Startup misfire recovery: check if the last scan was missed while
+        # the server was down.  If overdue by more than one interval but within
+        # the grace period, trigger an immediate rescan.
+        await self._check_missed_run()
+
     async def stop(self) -> None:
         """Stop the APScheduler gracefully."""
         logger.info("SchedulerService.stop() called")
@@ -303,6 +303,67 @@ class SchedulerService:
         )
         return trigger
 
+    async def _check_missed_run(self) -> None:
+        """Check if a scheduled rescan was missed while the server was down.
+
+        Compares system_settings.last_scan_timestamp against the expected
+        schedule.  If the last scan is overdue (more than 24h ago for a daily
+        schedule) but within the grace period, triggers an immediate rescan.
+        """
+        if not self._config or not self._config.enabled:
+            return
+        if not self._config.schedule_days:
+            return
+
+        try:
+            from src.server.database.connection import (  # noqa: PLC0415
+                get_db_session,
+            )
+            from src.server.database.system_settings_service import (  # noqa: PLC0415
+                SystemSettingsService,
+            )
+
+            async with get_db_session() as db:
+                settings = await SystemSettingsService.get_or_create(db)
+                last_scan = settings.last_scan_timestamp
+
+            if last_scan is None:
+                # Never scanned before — trigger immediately
+                logger.info("No previous scan recorded — triggering immediate rescan")
+                await self._perform_rescan()
+                return
+
+            # Ensure timezone-aware comparison
+            if last_scan.tzinfo is None:
+                last_scan = last_scan.replace(tzinfo=timezone.utc)
+
+            now = datetime.now(timezone.utc)
+            elapsed = now - last_scan
+
+            # If last scan was more than 24h + grace period ago, don't trigger
+            # (avoids surprise rescans after long downtime).
+            max_overdue = timedelta(hours=24, seconds=_MISFIRE_GRACE_SECONDS)
+            # If last scan was more than ~25h ago, skip (too stale)
+            if elapsed > max_overdue:
+                logger.info(
+                    "Last scan was %s ago (> %s) — skipping missed-run recovery",
+                    elapsed,
+                    max_overdue,
+                )
+                return
+
+            # Check if a run should have happened between last_scan and now.
+            # Simple heuristic: if elapsed > 24h, we missed at least one daily run.
+            if elapsed > timedelta(hours=23):
+                logger.info(
+                    "Missed scheduled rescan detected (last scan %s ago) — triggering now",
+                    elapsed,
+                )
+                await self._perform_rescan()
+
+        except Exception as exc:  # pylint: disable=broad-exception-caught
+            logger.warning("Missed-run check failed (non-fatal): %s", exc)
+
     async def _broadcast(self, event_type: str, data: dict) -> None:
         """Broadcast a WebSocket event to all connected clients."""
         try: