feat(settings): let users clear stored secrets from the UI

Redacted secrets (SMTP password, Telegram bot token, LDAP password) are
always served blank to the browser, so the update path treats a blank
submission as "unchanged" and silently restores the stored value. That
made a once-set secret impossible to remove without editing the database
— e.g. switching to a passwordless localhost SMTP relay kept sending the
old credentials forever.

Blank stays "unchanged"; clearing is now its own signal. The update
request carries explicit clear flags (request-scoped fields on the
controller form, so they are never persisted as settings rows), and
preserveRedactedSecrets skips the restore for a flagged secret. Each
secret field gets a Clear/Undo button that arms the flag; typing a new
value disarms it. The 2FA token keeps its existing behavior: it is
already clearable by disabling 2FA.

Closes #5724

internal/web/controller/setting.go

@@ -26,9 +26,16 @@ type updateUserForm struct {
 	TwoFactorCode string `json:"twoFactorCode" form:"twoFactorCode"`
 }
 
+// updateSettingForm carries the persisted settings plus request-scoped fields
+// that must never land in the settings table: the 2FA confirmation code and
+// the explicit clear flags for redacted secrets (a blank secret alone means
+// "unchanged", so clearing needs its own signal — see #5724).
 type updateSettingForm struct {
 	entity.AllSetting
-	TwoFactorCode string `json:"twoFactorCode" form:"twoFactorCode"`
+	TwoFactorCode     string `json:"twoFactorCode" form:"twoFactorCode"`
+	ClearTgBotToken   bool   `json:"clearTgBotToken" form:"clearTgBotToken"`
+	ClearLdapPassword bool   `json:"clearLdapPassword" form:"clearLdapPassword"`
+	ClearSmtpPassword bool   `json:"clearSmtpPassword" form:"clearSmtpPassword"`
 }
 
 // SettingController handles settings and user management operations.
@@ -105,7 +112,11 @@ func (a *SettingController) updateSetting(c *gin.Context) {
 			return
 		}
 	}
-	err := a.settingService.UpdateAllSetting(allSetting)
+	err := a.settingService.UpdateAllSetting(allSetting, service.SecretClears{
+		TgBotToken:   form.ClearTgBotToken,
+		LdapPassword: form.ClearLdapPassword,
+		SmtpPassword: form.ClearSmtpPassword,
+	})
 	if err == nil && twoFactorErr == nil && !oldTwoFactor && allSetting.TwoFactorEnable {
 		if bumpErr := a.userService.BumpLoginEpoch(); bumpErr != nil {
 			err = bumpErr

internal/web/service/setting.go

@@ -1085,8 +1085,17 @@ func (s *SettingService) SetSmtpMemory(value int) error {
 	return s.setInt("smtpMemory", value)
 }
 
-func (s *SettingService) UpdateAllSetting(allSetting *entity.AllSetting) error {
-	if err := s.preserveRedactedSecrets(allSetting); err != nil {
+// SecretClears marks redacted secrets the user explicitly emptied. Without a
+// flag, a blank submitted secret means "unchanged" (the field is always served
+// blank to the browser) and the stored value is preserved.
+type SecretClears struct {
+	TgBotToken   bool
+	LdapPassword bool
+	SmtpPassword bool
+}
+
+func (s *SettingService) UpdateAllSetting(allSetting *entity.AllSetting, clears SecretClears) error {
+	if err := s.preserveRedactedSecrets(allSetting, clears); err != nil {
 		return err
 	}
 	if err := validateSettingsURLs(allSetting); err != nil {
@@ -1132,15 +1141,15 @@ func (s *SettingService) UpdateAllSetting(allSetting *entity.AllSetting) error {
 	})
 }
 
-func (s *SettingService) preserveRedactedSecrets(allSetting *entity.AllSetting) error {
-	if strings.TrimSpace(allSetting.TgBotToken) == "" {
+func (s *SettingService) preserveRedactedSecrets(allSetting *entity.AllSetting, clears SecretClears) error {
+	if !clears.TgBotToken && strings.TrimSpace(allSetting.TgBotToken) == "" {
 		value, err := s.GetTgBotToken()
 		if err != nil {
 			return err
 		}
 		allSetting.TgBotToken = value
 	}
-	if strings.TrimSpace(allSetting.LdapPassword) == "" {
+	if !clears.LdapPassword && strings.TrimSpace(allSetting.LdapPassword) == "" {
 		value, err := s.GetLdapPassword()
 		if err != nil {
 			return err
@@ -1154,7 +1163,7 @@ func (s *SettingService) preserveRedactedSecrets(allSetting *entity.AllSetting)
 		}
 		allSetting.TwoFactorToken = value
 	}
-	if strings.TrimSpace(allSetting.SmtpPassword) == "" {
+	if !clears.SmtpPassword && strings.TrimSpace(allSetting.SmtpPassword) == "" {
 		value, err := s.GetSmtpPassword()
 		if err != nil {
 			return err

internal/web/service/setting_security_test.go

@@ -77,7 +77,7 @@ func TestUpdateAllSettingPreservesRedactedSecrets(t *testing.T) {
 		t.Fatal(err)
 	}
 	settings := &view.AllSetting
-	if err := s.UpdateAllSetting(settings); err != nil {
+	if err := s.UpdateAllSetting(settings, SecretClears{}); err != nil {
 		t.Fatal(err)
 	}
 	if got, _ := s.GetTgBotToken(); got != "telegram-secret" {
@@ -94,6 +94,54 @@ func TestUpdateAllSettingPreservesRedactedSecrets(t *testing.T) {
 	}
 }
 
+func TestUpdateAllSettingClearsFlaggedSecrets(t *testing.T) {
+	setupSettingTestDB(t)
+	s := &SettingService{}
+	if err := s.saveSetting("tgBotToken", "telegram-secret"); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.saveSetting("ldapPassword", "ldap-secret"); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.saveSetting("smtpPassword", "smtp-secret"); err != nil {
+		t.Fatal(err)
+	}
+
+	view, err := s.GetAllSettingView()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := s.UpdateAllSetting(&view.AllSetting, SecretClears{SmtpPassword: true}); err != nil {
+		t.Fatal(err)
+	}
+	if got, _ := s.GetSmtpPassword(); got != "" {
+		t.Fatalf("smtp password = %q, want cleared", got)
+	}
+	if got, _ := s.GetTgBotToken(); got != "telegram-secret" {
+		t.Fatalf("tg token = %q, unflagged secret must stay preserved", got)
+	}
+	if got, _ := s.GetLdapPassword(); got != "ldap-secret" {
+		t.Fatalf("ldap password = %q, unflagged secret must stay preserved", got)
+	}
+
+	view, err = s.GetAllSettingView()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if view.HasSmtpPassword {
+		t.Fatal("hasSmtpPassword must report false after clearing")
+	}
+	if err := s.UpdateAllSetting(&view.AllSetting, SecretClears{TgBotToken: true, LdapPassword: true}); err != nil {
+		t.Fatal(err)
+	}
+	if got, _ := s.GetTgBotToken(); got != "" {
+		t.Fatalf("tg token = %q, want cleared", got)
+	}
+	if got, _ := s.GetLdapPassword(); got != "" {
+		t.Fatalf("ldap password = %q, want cleared", got)
+	}
+}
+
 func TestSanitizePublicHTTPURLBlocksPrivateAddressUnlessAllowed(t *testing.T) {
 	if _, err := SanitizePublicHTTPURL("http://127.0.0.1:8080/hook", false); err == nil {
 		t.Fatal("expected localhost URL to be blocked")

internal/web/translation/ar-EG.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "عند تعيينه، يحل هذا محل نموذج الملاحظة لكل رابط اشتراك — اكتب صيغتك الخاصة باستخدام رموز المتغيرات (استخدم الزر لإدراجها). اتركه فارغاً لاستخدام النموذج أعلاه.",
       "validation": {
         "pathLeadingSlash": "يجب أن يبدأ المسار بالرمز /"
-      }
+      },
+      "secretClear": "مسح",
+      "secretClearUndo": "تراجع عن المسح"
     },
     "xray": {
       "title": "إعدادات Xray",

internal/web/translation/en-US.json

@@ -1515,7 +1515,9 @@
       "eventMemoryHigh": "Memory high (%)",
       "validation": {
         "pathLeadingSlash": "Path must start with /"
-      }
+      },
+      "secretClear": "Clear",
+      "secretClearUndo": "Undo clear"
     },
     "xray": {
       "title": "Xray Configs",

internal/web/translation/es-ES.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Cuando se define, esto reemplaza el modelo de notas para cada enlace de suscripción — escribe tu propio formato con los tokens de variable (usa el botón para insertarlos). Déjalo vacío para usar el modelo anterior.",
       "validation": {
         "pathLeadingSlash": "La ruta debe comenzar con /"
-      }
+      },
+      "secretClear": "Borrar",
+      "secretClearUndo": "Deshacer borrado"
     },
     "xray": {
       "title": "Xray Configuración",

internal/web/translation/fa-IR.json

@@ -1399,7 +1399,9 @@
       "eventMemoryHigh": "مصرف حافظه بالا (%)",
       "validation": {
         "pathLeadingSlash": "مسیر باید با / شروع شود"
-      }
+      },
+      "secretClear": "پاک کردن",
+      "secretClearUndo": "لغو پاک کردن"
     },
     "xray": {
       "title": "پیکربندی ایکس‌ری",

internal/web/translation/id-ID.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Jika diatur, ini menggantikan model catatan untuk setiap tautan langganan — tulis format Anda sendiri dengan token variabel (gunakan tombol untuk menyisipkannya). Biarkan kosong untuk memakai model di atas.",
       "validation": {
         "pathLeadingSlash": "Path harus diawali dengan /"
-      }
+      },
+      "secretClear": "Hapus",
+      "secretClearUndo": "Batalkan hapus"
     },
     "xray": {
       "title": "Konfigurasi Xray",

internal/web/translation/ja-JP.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "設定すると、すべてのサブスクリプションリンクの備考モデルを置き換えます — 変数トークンを使って独自の形式を記述してください（ボタンで挿入できます）。空欄にすると上記のモデルが使用されます。",
       "validation": {
         "pathLeadingSlash": "パスは / で始まる必要があります"
-      }
+      },
+      "secretClear": "クリア",
+      "secretClearUndo": "クリアを取り消す"
     },
     "xray": {
       "title": "Xray 設定",

internal/web/translation/pt-BR.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Quando definido, isto substitui o modelo de observação de cada link de assinatura — escreva seu próprio formato com os tokens de variáveis (use o botão para inseri-los). Deixe vazio para usar o modelo acima.",
       "validation": {
         "pathLeadingSlash": "O caminho deve começar com /"
-      }
+      },
+      "secretClear": "Limpar",
+      "secretClearUndo": "Desfazer limpeza"
     },
     "xray": {
       "title": "Configurações Xray",

internal/web/translation/ru-RU.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Если задан, заменяет модель примечания для каждой ссылки подписки — задайте собственный формат с помощью токенов переменных (используйте кнопку для их вставки). Оставьте пустым, чтобы использовать модель выше.",
       "validation": {
         "pathLeadingSlash": "Путь должен начинаться с /"
-      }
+      },
+      "secretClear": "Очистить",
+      "secretClearUndo": "Отменить очистку"
     },
     "xray": {
       "importRules": "Импорт правил",

internal/web/translation/tr-TR.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Ayarlandığında, her abonelik bağlantısının açıklama modelinin yerini alır — değişken belirteçleriyle kendi formatınızı yazın (eklemek için düğmeyi kullanın). Yukarıdaki modeli kullanmak için boş bırakın.",
       "validation": {
         "pathLeadingSlash": "Yol / ile başlamalıdır"
-      }
+      },
+      "secretClear": "Temizle",
+      "secretClearUndo": "Temizlemeyi geri al"
     },
     "xray": {
       "title": "Xray Yapılandırmaları",

internal/web/translation/uk-UA.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Якщо задано, це замінює модель примітки для кожного посилання підписки — напишіть власний формат із токенами змінних (використовуйте кнопку для їх вставлення). Залиште порожнім, щоб використовувати модель вище.",
       "validation": {
         "pathLeadingSlash": "Шлях має починатися з /"
-      }
+      },
+      "secretClear": "Очистити",
+      "secretClearUndo": "Скасувати очищення"
     },
     "xray": {
       "title": "Xray конфігурації",

internal/web/translation/vi-VN.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "Khi được đặt, mục này thay thế mô hình ghi chú cho mọi liên kết đăng ký — hãy viết định dạng riêng của bạn bằng các token biến (dùng nút để chèn chúng). Để trống để dùng mô hình ở trên.",
       "validation": {
         "pathLeadingSlash": "Đường dẫn phải bắt đầu bằng /"
-      }
+      },
+      "secretClear": "Xóa",
+      "secretClearUndo": "Hoàn tác xóa"
     },
     "xray": {
       "title": "Cài đặt Xray",

internal/web/translation/zh-CN.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "设置后，将替换每个订阅链接的备注模型 — 使用变量标记编写您自己的格式（用按钮插入它们）。留空则使用上方的模型。",
       "validation": {
         "pathLeadingSlash": "路径必须以 / 开头"
-      }
+      },
+      "secretClear": "清除",
+      "secretClearUndo": "撤销清除"
     },
     "xray": {
       "importRules": "导入规则",

internal/web/translation/zh-TW.json

@@ -1399,7 +1399,9 @@
       "remarkTemplateDesc": "設定後，這將取代每個訂閱連結的備註模型——使用變數標記撰寫您自己的格式（使用按鈕來插入）。留空則使用上方的模型。",
       "validation": {
         "pathLeadingSlash": "路徑必須以 / 開頭"
-      }
+      },
+      "secretClear": "清除",
+      "secretClearUndo": "復原清除"
     },
     "xray": {
       "title": "Xray 配置",