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/migrate/schema.go

@@ -1078,6 +1078,9 @@ var (
 		{Name: "totp_secret_encrypted", Type: field.TypeString, Nullable: true, SchemaType: map[string]string{"postgres": "text"}},
 		{Name: "totp_enabled", Type: field.TypeBool, Default: false},
 		{Name: "totp_enabled_at", Type: field.TypeTime, Nullable: true},
+		{Name: "balance_notify_enabled", Type: field.TypeBool, Default: true},
+		{Name: "balance_notify_threshold", Type: field.TypeFloat64, Nullable: true, SchemaType: map[string]string{"postgres": "decimal(20,8)"}},
+		{Name: "balance_notify_extra_emails", Type: field.TypeString, Default: "[]", SchemaType: map[string]string{"postgres": "text"}},
 	}
 	// UsersTable holds the schema information for the "users" table.
 	UsersTable = &schema.Table{