perf(网关): 复用分组上下文减少热路径查询

新增 GetByIDLite 并在网关与 Gemini 选择流程复用上下文 group，避免 COUNT 触发
更新 API key 中间件注入 group 上下文，减少重复查库
补充 gateway/gemini 中间件与仓库层回归测试

测试: make test

backend/internal/server/middleware/api_key_auth_test.go

@@ -11,6 +11,7 @@ import (
 	"time"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/ctxkey"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 	"github.com/gin-gonic/gin"
@@ -110,6 +111,63 @@ func TestSimpleModeBypassesQuotaCheck(t *testing.T) {
 	})
 }
 
+func TestAPIKeyAuthSetsGroupContext(t *testing.T) {
+	gin.SetMode(gin.TestMode)
+
+	group := &service.Group{
+		ID:       101,
+		Name:     "g1",
+		Status:   service.StatusActive,
+		Platform: service.PlatformAnthropic,
+	}
+	user := &service.User{
+		ID:          7,
+		Role:        service.RoleUser,
+		Status:      service.StatusActive,
+		Balance:     10,
+		Concurrency: 3,
+	}
+	apiKey := &service.APIKey{
+		ID:     100,
+		UserID: user.ID,
+		Key:    "test-key",
+		Status: service.StatusActive,
+		User:   user,
+		Group:  group,
+	}
+	apiKey.GroupID = &group.ID
+
+	apiKeyRepo := &stubApiKeyRepo{
+		getByKey: func(ctx context.Context, key string) (*service.APIKey, error) {
+			if key != apiKey.Key {
+				return nil, service.ErrAPIKeyNotFound
+			}
+			clone := *apiKey
+			return &clone, nil
+		},
+	}
+
+	cfg := &config.Config{RunMode: config.RunModeSimple}
+	apiKeyService := service.NewAPIKeyService(apiKeyRepo, nil, nil, nil, nil, cfg)
+	router := gin.New()
+	router.Use(gin.HandlerFunc(NewAPIKeyAuthMiddleware(apiKeyService, nil, cfg)))
+	router.GET("/t", func(c *gin.Context) {
+		groupFromCtx, ok := c.Request.Context().Value(ctxkey.Group).(*service.Group)
+		if !ok || groupFromCtx == nil || groupFromCtx.ID != group.ID {
+			c.JSON(http.StatusInternalServerError, gin.H{"ok": false})
+			return
+		}
+		c.JSON(http.StatusOK, gin.H{"ok": true})
+	})
+
+	w := httptest.NewRecorder()
+	req := httptest.NewRequest(http.MethodGet, "/t", nil)
+	req.Header.Set("x-api-key", apiKey.Key)
+	router.ServeHTTP(w, req)
+
+	require.Equal(t, http.StatusOK, w.Code)
+}
+
 func newAuthTestRouter(apiKeyService *service.APIKeyService, subscriptionService *service.SubscriptionService, cfg *config.Config) *gin.Engine {
 	router := gin.New()
 	router.Use(gin.HandlerFunc(NewAPIKeyAuthMiddleware(apiKeyService, subscriptionService, cfg)))