feat(notify): add balance low & account quota notification system

- User balance low notification: email alert when balance drops below
  configurable threshold (user email + verified extra emails)
- Account quota notification: broadcast email to admin-configured
  recipients when daily/weekly/total quota usage exceeds alert threshold
- Admin settings: global enable/disable, default threshold, quota
  notification email list (Email Settings tab)
- User profile: enable/disable, custom threshold, add/remove extra
  notification emails with verification code flow
- Account quota: per-dimension alert toggle and threshold in quota
  control card
- Trigger logic: first-crossing only (old >= threshold && new < threshold
  for balance; old < threshold && new >= threshold for quota), naturally
  prevents duplicate notifications without Redis dedup

backend/internal/service/settings_view.go

@@ -107,8 +107,12 @@ type SystemSettings struct {
 	EnableMetadataPassthrough    bool // 是否透传客户端原始 metadata（默认 false）
 	EnableCCHSigning             bool // 是否对 billing header cch 进行签名（默认 false）
 
-	// Web Search Emulation (read-only quick check; full config via dedicated API)
-	WebSearchEmulationEnabled bool
+	// Balance low notification
+	BalanceLowNotifyEnabled   bool
+	BalanceLowNotifyThreshold float64
+
+	// Account quota notification
+	AccountQuotaNotifyEmails []string
 }
 
 type DefaultSubscriptionSetting struct {
@@ -144,9 +148,9 @@ type PublicSettings struct {
 
 	LinuxDoOAuthEnabled   bool
 	BackendModeEnabled    bool
+	PaymentEnabled        bool
 	OIDCOAuthEnabled      bool
 	OIDCOAuthProviderName string
-	PaymentEnabled        bool
 	Version               string
 }