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/anime.py

@@ -20,7 +20,9 @@ from src.server.exceptions import (
 from src.server.models.anime import AnimeMetadataUpdate
 from src.server.services.anime_service import AnimeService, AnimeServiceError
 from src.server.services.background_loader_service import BackgroundLoaderService
-from src.server.services.folder_rename_service import _scan_for_pre_existing_duplicates
+from src.server.services.scheduler.folder_rename_service import (
+    _scan_for_pre_existing_duplicates,
+)
 from src.server.utils.dependencies import (
     get_anime_service,
     get_background_loader_service,