feat: add maintenance_schedules table, schema, and CRUD endpoints

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -23,6 +23,7 @@ from .feature_flag import FeatureFlag, PlanFeatureDefault, AccountFeatureOverrid
 from .platform_setting import PlatformSetting
 from .user_pinned_tree import UserPinnedTree
 from .target_list import TargetList
+from .maintenance_schedule import MaintenanceSchedule
 
 __all__ = [
     "User",
@@ -57,4 +58,5 @@ __all__ = [
     "PlatformSetting",
     "UserPinnedTree",
     "TargetList",
+    "MaintenanceSchedule",
 ]