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/services/scheduler/__init__.py

@@ -0,0 +1,45 @@
+"""Scheduler services package.
+
+Contains scheduler orchestration and rescan coordination:
+
+- scheduler_service: Cron-based scheduler using APScheduler
+- rescan_orchestrator: Legacy alias for RescanService (for backward compatibility)
+"""
+from __future__ import annotations
+
+from src.server.services.rescan_service import (
+                                                RescanService,
+                                                get_rescan_service,
+                                                reset_rescan_service,
+)
+
+# Backward compatibility alias
+from src.server.services.scheduler.rescan_orchestrator import (
+                                                RescanOrchestrator,
+                                                get_rescan_orchestrator,
+                                                reset_rescan_orchestrator,
+)
+from src.server.services.scheduler.scheduler_service import (
+                                                SchedulerService,
+                                                SchedulerServiceError,
+                                                get_scheduler_service,
+                                                reset_scheduler_service,
+)
+
+__all__ = [
+    # RescanService (new location)
+    "RescanService",
+    "get_rescan_service",
+    "reset_rescan_service",
+    # Scheduler
+    "SchedulerService",
+    "SchedulerServiceError",
+    "get_scheduler_service",
+    "reset_scheduler_service",
+    # Backward compatibility
+    "RescanOrchestrator",
+    "get_rescan_orchestrator",
+    "reset_rescan_orchestrator",
+    # Sub-services (still in scheduler folder)
+    "folder_rename_service",
+]