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/fastapi_app.py

@@ -411,7 +411,9 @@ async def lifespan(_application: FastAPI):
             # anime_directory may be configured there even if the env var is empty.
             try:
                 logger.info("Initializing scheduler service...")
-                from src.server.services.scheduler_service import get_scheduler_service
+                from src.server.services.scheduler.scheduler_service import (
+                    get_scheduler_service,
+                )
                 scheduler_service = get_scheduler_service()
                 logger.info("Scheduler service instance obtained, starting...")
                 await scheduler_service.start()
@@ -496,7 +498,9 @@ async def lifespan(_application: FastAPI):
     # 1. Stop scheduler service (only if initialized)
     if initialized['scheduler']:
         try:
-            from src.server.services.scheduler_service import get_scheduler_service
+            from src.server.services.scheduler.scheduler_service import (
+                get_scheduler_service,
+            )
             scheduler_service = get_scheduler_service()
             logger.info("Stopping scheduler service...")
             await asyncio.wait_for(