refactor(scheduler): separate scheduler logic from scan/rescan logic

- Extract rescan logic into new RescanService (src/server/services/rescan_service.py)
- SchedulerService now only handles APScheduler cron scheduling
- Move scheduler sub-services (folder_rename, folder_scan, key_resolution) to scheduler/ folder
- Keep RescanOrchestrator as backward-compatible alias
- Update all imports across api/, server/, and test files

src/server/api/health.py

@@ -195,7 +195,9 @@ async def basic_health_check(request: Request) -> HealthStatus:
     # Get scheduler status for health monitoring
     scheduler_status: dict = {}
     try:
-        from src.server.services.scheduler_service import get_scheduler_service
+        from src.server.services.scheduler.scheduler_service import (
+            get_scheduler_service,
+        )
         scheduler_status = get_scheduler_service().get_status()
     except Exception:
         pass