feat(rpm): RPM 限流模块优化

P0:
- rpm_override 嵌入 Auth Cache Snapshot，消除每请求 DB 查询 (snapshot v6→v7)
- 429 RPM 响应返回 Retry-After 头（当前分钟剩余秒数）

P1:
- ClearAll 按钮直连 DELETE API，带 loading 防重复
- 新增 GET /admin/users/:id/rpm-status 管理员 RPM 用量查询端点

优化:
- checkRPM 从级联互斥改为并行取最严，user.rpm_limit 作为全局硬上限始终生效
- Override/Group 变更后自动失效 auth cache
- fail-open 语义不变，Redis 故障不阻塞业务

backend/ent/user.go

@@ -61,6 +61,8 @@ type User struct {
 	BalanceNotifyExtraEmails string `json:"balance_notify_extra_emails,omitempty"`
 	// TotalRecharged holds the value of the "total_recharged" field.
 	TotalRecharged float64 `json:"total_recharged,omitempty"`
+	// RpmLimit holds the value of the "rpm_limit" field.
+	RpmLimit int `json:"rpm_limit,omitempty"`
 	// Edges holds the relations/edges for other nodes in the graph.
 	// The values are being populated by the UserQuery when eager-loading is set.
 	Edges        UserEdges `json:"edges"`
@@ -226,7 +228,7 @@ func (*User) scanValues(columns []string) ([]any, error) {
 			values[i] = new(sql.NullBool)
 		case user.FieldBalance, user.FieldBalanceNotifyThreshold, user.FieldTotalRecharged:
 			values[i] = new(sql.NullFloat64)
-		case user.FieldID, user.FieldConcurrency:
+		case user.FieldID, user.FieldConcurrency, user.FieldRpmLimit:
 			values[i] = new(sql.NullInt64)
 		case user.FieldEmail, user.FieldPasswordHash, user.FieldRole, user.FieldStatus, user.FieldUsername, user.FieldNotes, user.FieldTotpSecretEncrypted, user.FieldSignupSource, user.FieldBalanceNotifyThresholdType, user.FieldBalanceNotifyExtraEmails:
 			values[i] = new(sql.NullString)
@@ -391,6 +393,12 @@ func (_m *User) assignValues(columns []string, values []any) error {
 			} else if value.Valid {
 				_m.TotalRecharged = value.Float64
 			}
+		case user.FieldRpmLimit:
+			if value, ok := values[i].(*sql.NullInt64); !ok {
+				return fmt.Errorf("unexpected type %T for field rpm_limit", values[i])
+			} else if value.Valid {
+				_m.RpmLimit = int(value.Int64)
+			}
 		default:
 			_m.selectValues.Set(columns[i], values[i])
 		}
@@ -569,6 +577,9 @@ func (_m *User) String() string {
 	builder.WriteString(", ")
 	builder.WriteString("total_recharged=")
 	builder.WriteString(fmt.Sprintf("%v", _m.TotalRecharged))
+	builder.WriteString(", ")
+	builder.WriteString("rpm_limit=")
+	builder.WriteString(fmt.Sprintf("%v", _m.RpmLimit))
 	builder.WriteByte(')')
 	return builder.String()
 }