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/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))
+}