feat(notify): add percentage threshold type for balance low notification

- Add threshold_type field (fixed/percentage) to system and user settings
- Add total_recharged field to users table, auto-incremented on balance credit
- Percentage mode: effective threshold = total_recharged × percentage / 100
- User-level threshold_type inherits from system default when not set
- Update admin settings UI with radio selector (fixed amount / percentage)
- Migration: 102_add_balance_notify_threshold_type.sql

backend/ent/schema/user.go

@@ -76,6 +76,8 @@ func (User) Fields() []ent.Field {
 		// 余额不足通知
 		field.Bool("balance_notify_enabled").
 			Default(true),
+		field.String("balance_notify_threshold_type").
+			Default("fixed"), // "fixed" | "percentage"
 		field.Float("balance_notify_threshold").
 			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
 			Optional().
@@ -83,6 +85,9 @@ func (User) Fields() []ent.Field {
 		field.String("balance_notify_extra_emails").
 			SchemaType(map[string]string{dialect.Postgres: "text"}).
 			Default("[]"),
+		field.Float("total_recharged").
+			SchemaType(map[string]string{dialect.Postgres: "decimal(20,8)"}).
+			Default(0),
 	}
 }