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/scheduler.py

@@ -10,7 +10,7 @@ from fastapi import APIRouter, Depends, HTTPException, status
 
 from src.server.models.config import SchedulerConfig
 from src.server.services.config_service import ConfigServiceError, get_config_service
-from src.server.services.scheduler_service import get_scheduler_service
+from src.server.services.scheduler.scheduler_service import get_scheduler_service
 from src.server.utils.dependencies import require_auth
 
 logger = logging.getLogger(__name__)