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_google.go

@@ -63,6 +63,7 @@ func APIKeyAuthWithSubscriptionGoogle(apiKeyService *service.APIKeyService, subs
 				Concurrency: apiKey.User.Concurrency,
 			})
 			c.Set(string(ContextKeyUserRole), apiKey.User.Role)
+			setGroupContext(c, apiKey.Group)
 			c.Next()
 			return
 		}
@@ -102,6 +103,7 @@ func APIKeyAuthWithSubscriptionGoogle(apiKeyService *service.APIKeyService, subs
 			Concurrency: apiKey.User.Concurrency,
 		})
 		c.Set(string(ContextKeyUserRole), apiKey.User.Role)
+		setGroupContext(c, apiKey.Group)
 		c.Next()
 	}
 }