feat(storage): support deleting backends and improve admin UI

- Implement storage backend deletion, which automatically resets default storage settings and user-specific overrides when a backend is removed.
- Add unit tests covering the delete action and its cleanup side effects.
- Improve admin UI responsiveness, fixing table scrolling, flex wrapping, and text truncation for long storage backend names.
- Update security documentation to clarify trusted proxy configurations and explain how trusted proxies are protected from automatic bans.

backend/libs/services/auth.go

@@ -574,6 +574,38 @@ func (s *AuthService) SetUserStorageBackend(userID, backendID string) error {
 	return s.saveUser(user)
 }
 
+func (s *AuthService) ClearStorageBackendOverrides(backendID string) (int, error) {
+	backendID = strings.TrimSpace(backendID)
+	if backendID == "" {
+		return 0, nil
+	}
+	cleared := 0
+	err := s.db.Update(func(tx *bbolt.Tx) error {
+		users := tx.Bucket(usersBucket)
+		return users.ForEach(func(key, value []byte) error {
+			var user User
+			if err := json.Unmarshal(value, &user); err != nil {
+				return err
+			}
+			if user.Policy.StorageBackendID == nil || *user.Policy.StorageBackendID != backendID {
+				return nil
+			}
+			user.Policy.StorageBackendID = nil
+			user.UpdatedAt = time.Now().UTC()
+			next, err := json.Marshal(user)
+			if err != nil {
+				return err
+			}
+			if err := users.Put(key, next); err != nil {
+				return err
+			}
+			cleared++
+			return nil
+		})
+	})
+	return cleared, err
+}
+
 func (s *AuthService) UpdateUserAdminFields(userID, username, email, role, status string, policy UserPolicy) (User, error) {
 	if err := validateUserPolicy(policy); err != nil {
 		return User{}, err