feat: 允许管理员为持有有效订阅的用户绑定订阅类型分组

之前管理员无法通过 API 密钥管理将用户绑定到订阅类型分组（直接返回错误）。
现在改为检查用户是否持有该分组的有效订阅，有则允许绑定，无则拒绝。

- admin_service: 新增 userSubRepo 依赖，替换硬拒绝为订阅校验
- admin_service: 区分 ErrSubscriptionNotFound 和内部错误，避免 DB 故障被误报
- wire_gen/api_contract_test: 同步新增参数
- UserApiKeysModal: 管理员分组下拉不再过滤订阅类型分组

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/internal/server/api_contract_test.go

@@ -645,7 +645,7 @@ func newContractDeps(t *testing.T) *contractDeps {
 	settingRepo := newStubSettingRepo()
 	settingService := service.NewSettingService(settingRepo, cfg)
 
-	adminService := service.NewAdminService(userRepo, groupRepo, &accountRepo, nil, proxyRepo, apiKeyRepo, redeemRepo, nil, nil, nil, nil, nil, nil, nil, nil)
+	adminService := service.NewAdminService(userRepo, groupRepo, &accountRepo, nil, proxyRepo, apiKeyRepo, redeemRepo, nil, nil, nil, nil, nil, nil, nil, nil, nil)
 	authHandler := handler.NewAuthHandler(cfg, nil, userService, settingService, nil, redeemService, nil)
 	apiKeyHandler := handler.NewAPIKeyHandler(apiKeyService)
 	usageHandler := handler.NewUsageHandler(usageService, apiKeyService)