Fix scheduler next_run_time None check; add debug logging

- Fix race condition: next_run_time only available after scheduler.start()
- Handle None gracefully in logging
- Add debug logging to _perform_rescan and _run_rescan_job
- Document scheduler troubleshooting in DEVELOPMENT.md

src/server/services/scheduler_service.py

@@ -116,11 +116,13 @@ class SchedulerService:
         # 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()
         job = self._scheduler.get_job(_JOB_ID)
-        if job and job.next_run_time:
+        if job:
+            next_run = job.next_run_time
             logger.info(
                 "Scheduler next run",
-                next_run=job.next_run_time.isoformat(),
+                next_run=next_run.isoformat() if next_run else None,
             )
 
     async def stop(self) -> None:
@@ -349,6 +351,7 @@ class SchedulerService:
 
     async def _perform_rescan(self) -> None:
         """Execute a library rescan and optionally trigger auto-download."""
+        logger.info("Scheduler _perform_rescan entered", scan_in_progress=self._scan_in_progress)
         if self._scan_in_progress:
             logger.warning("Skipping rescan: previous scan still in progress")
             return
@@ -443,6 +446,7 @@ class SchedulerService:
 
 async def _run_rescan_job() -> None:
     """Module-level job entry point — delegates to the current service."""
+    logger.info("APScheduler triggered _run_rescan_job")
     svc = get_scheduler_service()
     await svc._perform_rescan()