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

tests/api/test_config_endpoints.py

@@ -216,7 +216,7 @@ async def test_update_config_with_anime_directory_starts_scheduler(
     """PUT /api/config with anime_directory syncs and starts scheduler."""
     mock_scheduler = AsyncMock()
     
-    with patch("src.server.services.scheduler_service.get_scheduler_service") as mock_sched_fn:
+    with patch("src.server.services.scheduler.scheduler_service.get_scheduler_service") as mock_sched_fn:
         mock_sched_fn.return_value = mock_scheduler
 
         with patch("src.config.settings.settings") as mock_settings:
@@ -238,7 +238,7 @@ async def test_update_config_without_anime_directory_does_not_start_scheduler(
     """PUT /api/config without new anime_directory does not call scheduler.ensure_started()."""
     mock_scheduler = AsyncMock()
     
-    with patch("src.server.services.scheduler_service.get_scheduler_service") as mock_sched_fn:
+    with patch("src.server.services.scheduler.scheduler_service.get_scheduler_service") as mock_sched_fn:
         mock_sched_fn.return_value = mock_scheduler
 
         with patch("src.config.settings.settings") as mock_settings: