feat(notify): convert email lists to NotifyEmailEntry struct with toggle support

- Change balance_notify_extra_emails and account_quota_notify_emails
  from []string to []NotifyEmailEntry{email, disabled, verified}
- Add per-email enable/disable toggle for both user and admin notifications
- Add PUT /user/notify-email/toggle API endpoint
- Fix critical bug: API key auth cache snapshot missing balance notify
  fields (Email, Username, BalanceNotifyEnabled, etc.), causing
  notifications to never fire on cached request paths
- Bump cache snapshot version 3→4 to invalidate stale entries
- Add SQL migration 104 to convert old format data
- Backward compatible: parseNotifyEmails auto-detects old/new format
- User balance notify: max 3 emails (primary + 2 extra)
- Admin quota notify: unlimited emails, each with toggle

backend/internal/handler/admin/setting_handler.go

@@ -178,7 +178,7 @@ func (h *SettingHandler) GetSettings(c *gin.Context) {
 		WebSearchEmulationEnabled:            settings.WebSearchEmulationEnabled,
 		BalanceLowNotifyEnabled:              settings.BalanceLowNotifyEnabled,
 		BalanceLowNotifyThreshold:            settings.BalanceLowNotifyThreshold,
-		AccountQuotaNotifyEmails:             settings.AccountQuotaNotifyEmails,
+		AccountQuotaNotifyEmails:             dto.NotifyEmailEntriesFromService(settings.AccountQuotaNotifyEmails),
 		PaymentEnabled:                       paymentCfg.Enabled,
 		PaymentMinAmount:                     paymentCfg.MinAmount,
 		PaymentMaxAmount:                     paymentCfg.MaxAmount,
@@ -311,7 +311,7 @@ type UpdateSettingsRequest struct {
 	// Balance low notification
 	BalanceLowNotifyEnabled   *bool     `json:"balance_low_notify_enabled"`
 	BalanceLowNotifyThreshold *float64  `json:"balance_low_notify_threshold"`
-	AccountQuotaNotifyEmails  *[]string `json:"account_quota_notify_emails"`
+	AccountQuotaNotifyEmails  *[]dto.NotifyEmailEntry `json:"account_quota_notify_emails"`
 
 	// Payment configuration (integrated into settings, full replace)
 	PaymentEnabled           *bool    `json:"payment_enabled"`
@@ -902,9 +902,9 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 			}
 			return previousSettings.BalanceLowNotifyThreshold
 		}(),
-		AccountQuotaNotifyEmails: func() []string {
+		AccountQuotaNotifyEmails: func() []service.NotifyEmailEntry {
 			if req.AccountQuotaNotifyEmails != nil {
-				return *req.AccountQuotaNotifyEmails
+				return dto.NotifyEmailEntriesToService(*req.AccountQuotaNotifyEmails)
 			}
 			return previousSettings.AccountQuotaNotifyEmails
 		}(),
@@ -1056,7 +1056,7 @@ func (h *SettingHandler) UpdateSettings(c *gin.Context) {
 		EnableCCHSigning:                     updatedSettings.EnableCCHSigning,
 		BalanceLowNotifyEnabled:              updatedSettings.BalanceLowNotifyEnabled,
 		BalanceLowNotifyThreshold:            updatedSettings.BalanceLowNotifyThreshold,
-		AccountQuotaNotifyEmails:             updatedSettings.AccountQuotaNotifyEmails,
+		AccountQuotaNotifyEmails:             dto.NotifyEmailEntriesFromService(updatedSettings.AccountQuotaNotifyEmails),
 		PaymentEnabled:                       updatedPaymentCfg.Enabled,
 		PaymentMinAmount:                     updatedPaymentCfg.MinAmount,
 		PaymentMaxAmount:                     updatedPaymentCfg.MaxAmount,

backend/internal/handler/dto/mappers.go

@@ -26,7 +26,7 @@ func UserFromServiceShallow(u *service.User) *User {
 		BalanceNotifyEnabled:       u.BalanceNotifyEnabled,
 		BalanceNotifyThresholdType: u.BalanceNotifyThresholdType,
 		BalanceNotifyThreshold:     u.BalanceNotifyThreshold,
-		BalanceNotifyExtraEmails:   u.BalanceNotifyExtraEmails,
+		BalanceNotifyExtraEmails:   NotifyEmailEntriesFromService(u.BalanceNotifyExtraEmails),
 		TotalRecharged:             u.TotalRecharged,
 	}
 }

backend/internal/handler/dto/notify_email_entry.go

@@ -0,0 +1,43 @@
+package dto
+
+import "github.com/Wei-Shaw/sub2api/internal/service"
+
+// NotifyEmailEntry represents a notification email with enable/disable and verification state.
+// Email="" is a placeholder for the "primary email" (user's registration email or first admin email).
+type NotifyEmailEntry struct {
+	Email    string `json:"email"`
+	Disabled bool   `json:"disabled"`
+	Verified bool   `json:"verified"`
+}
+
+// NotifyEmailEntriesFromService converts service entries to DTO entries.
+func NotifyEmailEntriesFromService(entries []service.NotifyEmailEntry) []NotifyEmailEntry {
+	if entries == nil {
+		return nil
+	}
+	result := make([]NotifyEmailEntry, len(entries))
+	for i, e := range entries {
+		result[i] = NotifyEmailEntry{
+			Email:    e.Email,
+			Disabled: e.Disabled,
+			Verified: e.Verified,
+		}
+	}
+	return result
+}
+
+// NotifyEmailEntriesToService converts DTO entries to service entries.
+func NotifyEmailEntriesToService(entries []NotifyEmailEntry) []service.NotifyEmailEntry {
+	if entries == nil {
+		return nil
+	}
+	result := make([]service.NotifyEmailEntry, len(entries))
+	for i, e := range entries {
+		result[i] = service.NotifyEmailEntry{
+			Email:    e.Email,
+			Disabled: e.Disabled,
+			Verified: e.Verified,
+		}
+	}
+	return result
+}

backend/internal/handler/dto/settings.go

@@ -152,7 +152,7 @@ type SystemSettings struct {
 	// Balance low notification
 	BalanceLowNotifyEnabled   bool     `json:"balance_low_notify_enabled"`
 	BalanceLowNotifyThreshold float64  `json:"balance_low_notify_threshold"`
-	AccountQuotaNotifyEmails  []string `json:"account_quota_notify_emails"`
+	AccountQuotaNotifyEmails  []NotifyEmailEntry `json:"account_quota_notify_emails"`
 }
 
 type DefaultSubscriptionSetting struct {

backend/internal/handler/dto/types.go

@@ -22,7 +22,7 @@ type User struct {
 	BalanceNotifyEnabled       bool     `json:"balance_notify_enabled"`
 	BalanceNotifyThresholdType string   `json:"balance_notify_threshold_type"`
 	BalanceNotifyThreshold     *float64 `json:"balance_notify_threshold"`
-	BalanceNotifyExtraEmails   []string `json:"balance_notify_extra_emails"`
+	BalanceNotifyExtraEmails   []NotifyEmailEntry `json:"balance_notify_extra_emails"`
 	TotalRecharged             float64  `json:"total_recharged"`
 
 	APIKeys       []APIKey           `json:"api_keys,omitempty"`

backend/internal/handler/user_handler.go

@@ -214,3 +214,39 @@ func (h *UserHandler) RemoveNotifyEmail(c *gin.Context) {
 
 	response.Success(c, dto.UserFromService(updatedUser))
 }
+
+// ToggleNotifyEmailRequest represents the request to toggle a notify email's disabled state
+type ToggleNotifyEmailRequest struct {
+	Email    string `json:"email"`    // empty string for primary email placeholder
+	Disabled bool   `json:"disabled"`
+}
+
+// ToggleNotifyEmail toggles the disabled state of a notification email
+// PUT /api/v1/user/notify-email/toggle
+func (h *UserHandler) ToggleNotifyEmail(c *gin.Context) {
+	subject, ok := middleware2.GetAuthSubjectFromContext(c)
+	if !ok {
+		response.Unauthorized(c, "User not authenticated")
+		return
+	}
+
+	var req ToggleNotifyEmailRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		response.BadRequest(c, "Invalid request: "+err.Error())
+		return
+	}
+
+	err := h.userService.ToggleNotifyEmail(c.Request.Context(), subject.UserID, req.Email, req.Disabled)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	updatedUser, err := h.userService.GetByID(c.Request.Context(), subject.UserID)
+	if err != nil {
+		response.ErrorFrom(c, err)
+		return
+	}
+
+	response.Success(c, dto.UserFromService(updatedUser))
+}