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/config.py

@@ -57,7 +57,7 @@ async def update_config(
         # Start scheduler if anime_directory was just configured
         if anime_dir_changed:
             try:
-                from src.server.services.scheduler_service import (
+                from src.server.services.scheduler.scheduler_service import (
                     get_scheduler_service,
                 )