fix: resolve all P0/P1 code quality issues

P0 fixes:
- P0-01: sticky_session_test.go add context import
- P0-02: wire_gen.go add usageLogRepository parameter
- P0-03: admin_service_stub_test.go add GetGroupAPIKeyCount
- P0-04: admin_basic_handlers_test.go add stubUsageLogRepository

P1 fixes:
- P1-03: group_handler.go GetStats implement real data query

E2E fixes:
- Fix API Key path to /api/v1/keys (user endpoint)

Documentation:
- Update MEMORY.md with latest fixes

backend/internal/handler/admin/admin_service_stub_test.go

@@ -6,6 +6,8 @@ import (
 	"sync"
 	"time"
 
+	"github.com/Wei-Shaw/sub2api/internal/pkg/pagination"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/usagestats"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 )
 
@@ -175,6 +177,10 @@ func (s *stubAdminService) GetGroupAPIKeys(ctx context.Context, groupID int64, p
 	return s.apiKeys, int64(len(s.apiKeys)), nil
 }
 
+func (s *stubAdminService) GetGroupAPIKeyCount(_ context.Context, _ int64) (int64, error) {
+	return int64(len(s.apiKeys)), nil
+}
+
 func (s *stubAdminService) GetGroupRateMultipliers(_ context.Context, _ int64) ([]service.UserGroupRateEntry, error) {
 	return nil, nil
 }
@@ -451,3 +457,120 @@ func (s *stubAdminService) ReplaceUserGroup(ctx context.Context, userID, oldGrou
 
 // Ensure stub implements interface.
 var _ service.AdminService = (*stubAdminService)(nil)
+var _ service.UsageLogRepository = (*stubUsageLogRepository)(nil)
+
+// stubUsageLogRepository is a minimal no-op stub for service.UsageLogRepository used in handler tests.
+type stubUsageLogRepository struct{}
+
+func (s *stubUsageLogRepository) Create(ctx context.Context, log *service.UsageLog) (bool, error) {
+	return true, nil
+}
+func (s *stubUsageLogRepository) GetByID(ctx context.Context, id int64) (*service.UsageLog, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) Delete(ctx context.Context, id int64) error { return nil }
+func (s *stubUsageLogRepository) ListByUser(ctx context.Context, userID int64, params pagination.PaginationParams) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) ListByAPIKey(ctx context.Context, apiKeyID int64, params pagination.PaginationParams) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) ListByAccount(ctx context.Context, accountID int64, params pagination.PaginationParams) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) ListByUserAndTimeRange(ctx context.Context, userID int64, startTime, endTime time.Time) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) ListByAPIKeyAndTimeRange(ctx context.Context, apiKeyID int64, startTime, endTime time.Time) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) ListByAccountAndTimeRange(ctx context.Context, accountID int64, startTime, endTime time.Time) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) ListByModelAndTimeRange(ctx context.Context, modelName string, startTime, endTime time.Time) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) GetAccountWindowStats(ctx context.Context, accountID int64, startTime time.Time) (*usagestats.AccountStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAccountTodayStats(ctx context.Context, accountID int64) (*usagestats.AccountStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetDashboardStats(ctx context.Context) (*usagestats.DashboardStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUsageTrendWithFilters(ctx context.Context, startTime, endTime time.Time, granularity string, userID, apiKeyID, accountID, groupID int64, model string, requestType *int16, stream *bool, billingType *int8) ([]usagestats.TrendDataPoint, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetModelStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, requestType *int16, stream *bool, billingType *int8) ([]usagestats.ModelStat, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetEndpointStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, model string, requestType *int16, stream *bool, billingType *int8) ([]usagestats.EndpointStat, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUpstreamEndpointStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, model string, requestType *int16, stream *bool, billingType *int8) ([]usagestats.EndpointStat, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetGroupStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, requestType *int16, stream *bool, billingType *int8) ([]usagestats.GroupStat, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) ([]usagestats.UserBreakdownItem, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAllGroupUsageSummary(ctx context.Context, todayStart time.Time) ([]usagestats.GroupUsageSummary, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.UserUsageTrendPoint, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserSpendingRanking(ctx context.Context, startTime, endTime time.Time, limit int) (*usagestats.UserSpendingRankingResponse, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetBatchUserUsageStats(ctx context.Context, userIDs []int64, startTime, endTime time.Time) (map[int64]*usagestats.BatchUserUsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetBatchAPIKeyUsageStats(ctx context.Context, apiKeyIDs []int64, startTime, endTime time.Time) (map[int64]*usagestats.BatchAPIKeyUsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserDashboardStats(ctx context.Context, userID int64) (*usagestats.UserDashboardStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAPIKeyDashboardStats(ctx context.Context, apiKeyID int64) (*usagestats.UserDashboardStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserUsageTrendByUserID(ctx context.Context, userID int64, startTime, endTime time.Time, granularity string) ([]usagestats.TrendDataPoint, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserModelStats(ctx context.Context, userID int64, startTime, endTime time.Time) ([]usagestats.ModelStat, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) ListWithFilters(ctx context.Context, params pagination.PaginationParams, filters usagestats.UsageLogFilters) ([]service.UsageLog, *pagination.PaginationResult, error) {
+	return nil, nil, nil
+}
+func (s *stubUsageLogRepository) GetGlobalStats(ctx context.Context, startTime, endTime time.Time) (*usagestats.UsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetStatsWithFilters(ctx context.Context, filters usagestats.UsageLogFilters) (*usagestats.UsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAccountUsageStats(ctx context.Context, accountID int64, startTime, endTime time.Time) (*usagestats.AccountUsageStatsResponse, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetUserStatsAggregated(ctx context.Context, userID int64, startTime, endTime time.Time) (*usagestats.UsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAPIKeyStatsAggregated(ctx context.Context, apiKeyID int64, startTime, endTime time.Time) (*usagestats.UsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetAccountStatsAggregated(ctx context.Context, accountID int64, startTime, endTime time.Time) (*usagestats.UsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetModelStatsAggregated(ctx context.Context, modelName string, startTime, endTime time.Time) (*usagestats.UsageStats, error) {
+	return nil, nil
+}
+func (s *stubUsageLogRepository) GetDailyStatsAggregated(ctx context.Context, userID int64, startTime, endTime time.Time) ([]map[string]any, error) {
+	return nil, nil
+}