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.go

@@ -1,11 +1,13 @@
 package middleware
 
 import (
+	"context"
 	"errors"
 	"log"
 	"strings"
 
 	"github.com/Wei-Shaw/sub2api/internal/config"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/ctxkey"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
 	"github.com/gin-gonic/gin"
@@ -91,6 +93,7 @@ func apiKeyAuthWithSubscription(apiKeyService *service.APIKeyService, subscripti
 				Concurrency: apiKey.User.Concurrency,
 			})
 			c.Set(string(ContextKeyUserRole), apiKey.User.Role)
+			setGroupContext(c, apiKey.Group)
 			c.Next()
 			return
 		}
@@ -149,6 +152,7 @@ func apiKeyAuthWithSubscription(apiKeyService *service.APIKeyService, subscripti
 			Concurrency: apiKey.User.Concurrency,
 		})
 		c.Set(string(ContextKeyUserRole), apiKey.User.Role)
+		setGroupContext(c, apiKey.Group)
 
 		c.Next()
 	}
@@ -173,3 +177,14 @@ func GetSubscriptionFromContext(c *gin.Context) (*service.UserSubscription, bool
 	subscription, ok := value.(*service.UserSubscription)
 	return subscription, ok
 }
+
+func setGroupContext(c *gin.Context, group *service.Group) {
+	if group == nil {
+		return
+	}
+	if existing, ok := c.Request.Context().Value(ctxkey.Group).(*service.Group); ok && existing != nil && existing.ID == group.ID {
+		return
+	}
+	ctx := context.WithValue(c.Request.Context(), ctxkey.Group, group)
+	c.Request = c.Request.WithContext(ctx)
+}