feat: add feedback database model and migration

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -25,6 +25,7 @@ from .platform_setting import PlatformSetting
 from .user_pinned_tree import UserPinnedTree
 from .target_list import TargetList
 from .maintenance_schedule import MaintenanceSchedule
+from .feedback import Feedback
 
 __all__ = [
     "User",
@@ -61,4 +62,5 @@ __all__ = [
     "UserPinnedTree",
     "TargetList",
     "MaintenanceSchedule",
+    "Feedback",
 ]