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/unit/test_concurrent_scans.py

@@ -474,7 +474,7 @@ class TestSchedulerConcurrentScanPrevention:
     @pytest.mark.asyncio
     async def test_scheduler_skips_rescan_if_already_running(self):
         """Test scheduler skips scheduled rescan if one is already running."""
-        from src.server.services.scheduler_service import SchedulerService
+        from src.server.services.scheduler.scheduler_service import SchedulerService
         
         scheduler = SchedulerService()
         
@@ -495,7 +495,7 @@ class TestSchedulerConcurrentScanPrevention:
     @pytest.mark.asyncio
     async def test_scheduler_sets_flag_during_rescan(self):
         """Test that scheduler properly sets scan_in_progress flag."""
-        from src.server.services.scheduler_service import SchedulerService
+        from src.server.services.scheduler.scheduler_service import SchedulerService
         
         scheduler = SchedulerService()