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/auth.py

@@ -165,7 +165,7 @@ async def setup_auth(req: SetupRequest):
                 
                 # Start scheduler if anime_directory is now set
                 try:
-                    from src.server.services.scheduler_service import (
+                    from src.server.services.scheduler.scheduler_service import (
                         get_scheduler_service,
                     )