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/internal/service/billing_cache_service_singleflight_test.go

@@ -100,7 +100,7 @@ func TestBillingCacheServiceGetUserBalance_Singleflight(t *testing.T) {
 		delay:   80 * time.Millisecond,
 		balance: 12.34,
 	}
-	svc := NewBillingCacheService(cache, userRepo, nil, nil, &config.Config{})
+	svc := NewBillingCacheService(cache, userRepo, nil, nil, nil, nil, &config.Config{})
 	t.Cleanup(svc.Stop)
 
 	const goroutines = 16