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/ent/schema/user.go

@@ -72,6 +72,17 @@ func (User) Fields() []ent.Field {
 		field.Time("totp_enabled_at").
 			Optional().
 			Nillable(),
+
+		// 余额不足通知
+		field.Bool("balance_notify_enabled").
+			Default(true),
+		field.Float("balance_notify_threshold").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
+			Optional().
+			Nillable(),
+		field.String("balance_notify_extra_emails").
+			SchemaType(map[string]string{dialect.Postgres: "text"}).
+			Default("[]"),
 	}
 }