feat(dashboard): add per-user drill-down for group, model, and endpoint distributions

Click on a group name, model name, or endpoint name in the distribution
tables to expand and show per-user usage breakdown (requests, tokens,
actual cost, standard cost).

Backend: new GET /admin/dashboard/user-breakdown API with group_id,
model, endpoint, endpoint_type filters.
Frontend: clickable rows with expand/collapse sub-table in all three
distribution charts.

backend/internal/handler/admin/dashboard_handler.go

@@ -9,6 +9,7 @@ import (
 
 	"github.com/Wei-Shaw/sub2api/internal/pkg/response"
 	"github.com/Wei-Shaw/sub2api/internal/pkg/timezone"
+	"github.com/Wei-Shaw/sub2api/internal/pkg/usagestats"
 	"github.com/Wei-Shaw/sub2api/internal/service"
 
 	"github.com/gin-gonic/gin"
@@ -604,3 +605,41 @@ func (h *DashboardHandler) GetBatchAPIKeysUsage(c *gin.Context) {
 	c.Header("X-Snapshot-Cache", "miss")
 	response.Success(c, payload)
 }
+
+// GetUserBreakdown handles getting per-user usage breakdown within a dimension.
+// GET /api/v1/admin/dashboard/user-breakdown
+// Query params: start_date, end_date, group_id, model, endpoint, endpoint_type, limit
+func (h *DashboardHandler) GetUserBreakdown(c *gin.Context) {
+	startTime, endTime := parseTimeRange(c)
+
+	dim := usagestats.UserBreakdownDimension{}
+	if v := c.Query("group_id"); v != "" {
+		if id, err := strconv.ParseInt(v, 10, 64); err == nil {
+			dim.GroupID = id
+		}
+	}
+	dim.Model = c.Query("model")
+	dim.Endpoint = c.Query("endpoint")
+	dim.EndpointType = c.DefaultQuery("endpoint_type", "inbound")
+
+	limit := 50
+	if v := c.Query("limit"); v != "" {
+		if n, err := strconv.Atoi(v); err == nil && n > 0 && n <= 200 {
+			limit = n
+		}
+	}
+
+	stats, err := h.dashboardService.GetUserBreakdownStats(
+		c.Request.Context(), startTime, endTime, dim, limit,
+	)
+	if err != nil {
+		response.Error(c, 500, "Failed to get user breakdown stats")
+		return
+	}
+
+	response.Success(c, gin.H{
+		"users":      stats,
+		"start_date": startTime.Format("2006-01-02"),
+		"end_date":   endTime.Add(-24 * time.Hour).Format("2006-01-02"),
+	})
+}

backend/internal/handler/sora_gateway_handler_test.go

@@ -345,6 +345,9 @@ func (s *stubUsageLogRepo) GetUpstreamEndpointStatsWithFilters(ctx context.Conte
 func (s *stubUsageLogRepo) 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 *stubUsageLogRepo) GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) ([]usagestats.UserBreakdownItem, error) {
+	return nil, nil
+}
 func (s *stubUsageLogRepo) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
 	return nil, nil
 }

backend/internal/pkg/usagestats/usage_log_types.go

@@ -129,6 +129,24 @@ type UserSpendingRankingResponse struct {
 	TotalTokens     int64                     `json:"total_tokens"`
 }
 
+// UserBreakdownItem represents per-user usage breakdown within a dimension (group, model, endpoint).
+type UserBreakdownItem struct {
+	UserID      int64   `json:"user_id"`
+	Email       string  `json:"email"`
+	Requests    int64   `json:"requests"`
+	TotalTokens int64   `json:"total_tokens"`
+	Cost        float64 `json:"cost"`        // 标准计费
+	ActualCost  float64 `json:"actual_cost"` // 实际扣除
+}
+
+// UserBreakdownDimension specifies the dimension to filter for user breakdown.
+type UserBreakdownDimension struct {
+	GroupID      int64  // filter by group_id (>0 to enable)
+	Model        string // filter by model name (non-empty to enable)
+	Endpoint     string // filter by endpoint value (non-empty to enable)
+	EndpointType string // "inbound", "upstream", or "path"
+}
+
 // APIKeyUsageTrendPoint represents API key usage trend data point
 type APIKeyUsageTrendPoint struct {
 	Date     string `json:"date"`

backend/internal/repository/usage_log_repo.go

@@ -3000,6 +3000,85 @@ func (r *usageLogRepository) GetGroupStatsWithFilters(ctx context.Context, start
 	return results, nil
 }
 
+// GetUserBreakdownStats returns per-user usage breakdown within a specific dimension.
+func (r *usageLogRepository) GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) (results []usagestats.UserBreakdownItem, err error) {
+	query := `
+		SELECT
+			COALESCE(ul.user_id, 0) as user_id,
+			COALESCE(u.email, '') as email,
+			COUNT(*) as requests,
+			COALESCE(SUM(ul.input_tokens + ul.output_tokens + ul.cache_creation_tokens + ul.cache_read_tokens), 0) as total_tokens,
+			COALESCE(SUM(ul.total_cost), 0) as cost,
+			COALESCE(SUM(ul.actual_cost), 0) as actual_cost
+		FROM usage_logs ul
+		LEFT JOIN users u ON u.id = ul.user_id
+		WHERE ul.created_at >= $1 AND ul.created_at < $2
+	`
+	args := []any{startTime, endTime}
+
+	if dim.GroupID > 0 {
+		query += fmt.Sprintf(" AND ul.group_id = $%d", len(args)+1)
+		args = append(args, dim.GroupID)
+	}
+	if dim.Model != "" {
+		query += fmt.Sprintf(" AND ul.model = $%d", len(args)+1)
+		args = append(args, dim.Model)
+	}
+	if dim.Endpoint != "" {
+		col := resolveEndpointColumn(dim.EndpointType)
+		query += fmt.Sprintf(" AND %s = $%d", col, len(args)+1)
+		args = append(args, dim.Endpoint)
+	}
+
+	query += " GROUP BY ul.user_id, u.email ORDER BY actual_cost DESC"
+	if limit > 0 {
+		query += fmt.Sprintf(" LIMIT %d", limit)
+	}
+
+	rows, err := r.sql.QueryContext(ctx, query, args...)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		if closeErr := rows.Close(); closeErr != nil && err == nil {
+			err = closeErr
+			results = nil
+		}
+	}()
+
+	results = make([]usagestats.UserBreakdownItem, 0)
+	for rows.Next() {
+		var row usagestats.UserBreakdownItem
+		if err := rows.Scan(
+			&row.UserID,
+			&row.Email,
+			&row.Requests,
+			&row.TotalTokens,
+			&row.Cost,
+			&row.ActualCost,
+		); err != nil {
+			return nil, err
+		}
+		results = append(results, row)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+	return results, nil
+}
+
+// resolveEndpointColumn maps endpoint type to the corresponding DB column name.
+func resolveEndpointColumn(endpointType string) string {
+	switch endpointType {
+	case "upstream":
+		return "ul.upstream_endpoint"
+	case "path":
+		return "ul.inbound_endpoint || ' -> ' || ul.upstream_endpoint"
+	default:
+		return "ul.inbound_endpoint"
+	}
+}
+
 // GetGlobalStats gets usage statistics for all users within a time range
 func (r *usageLogRepository) GetGlobalStats(ctx context.Context, startTime, endTime time.Time) (*UsageStats, error) {
 	query := `

backend/internal/server/api_contract_test.go

@@ -1637,6 +1637,10 @@ func (r *stubUsageLogRepo) GetGroupStatsWithFilters(ctx context.Context, startTi
 	return nil, errors.New("not implemented")
 }
 
+func (r *stubUsageLogRepo) GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) ([]usagestats.UserBreakdownItem, error) {
+	return nil, errors.New("not implemented")
+}
+
 func (r *stubUsageLogRepo) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
 	return nil, errors.New("not implemented")
 }

backend/internal/server/routes/admin.go

@@ -198,6 +198,7 @@ func registerDashboardRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		dashboard.GET("/users-ranking", h.Admin.Dashboard.GetUserSpendingRanking)
 		dashboard.POST("/users-usage", h.Admin.Dashboard.GetBatchUsersUsage)
 		dashboard.POST("/api-keys-usage", h.Admin.Dashboard.GetBatchAPIKeysUsage)
+		dashboard.GET("/user-breakdown", h.Admin.Dashboard.GetUserBreakdown)
 		dashboard.POST("/aggregation/backfill", h.Admin.Dashboard.BackfillAggregation)
 	}
 }

backend/internal/service/account_usage_service.go

@@ -48,6 +48,7 @@ type UsageLogRepository interface {
 	GetEndpointStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, model string, requestType *int16, stream *bool, billingType *int8) ([]usagestats.EndpointStat, error)
 	GetUpstreamEndpointStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, model string, requestType *int16, stream *bool, billingType *int8) ([]usagestats.EndpointStat, error)
 	GetGroupStatsWithFilters(ctx context.Context, startTime, endTime time.Time, userID, apiKeyID, accountID, groupID int64, requestType *int16, stream *bool, billingType *int8) ([]usagestats.GroupStat, error)
+	GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) ([]usagestats.UserBreakdownItem, error)
 	GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error)
 	GetUserUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.UserUsageTrendPoint, error)
 	GetUserSpendingRanking(ctx context.Context, startTime, endTime time.Time, limit int) (*usagestats.UserSpendingRankingResponse, error)

backend/internal/service/dashboard_service.go

@@ -335,6 +335,14 @@ func (s *DashboardService) GetUserSpendingRanking(ctx context.Context, startTime
 	return ranking, nil
 }
 
+func (s *DashboardService) GetUserBreakdownStats(ctx context.Context, startTime, endTime time.Time, dim usagestats.UserBreakdownDimension, limit int) ([]usagestats.UserBreakdownItem, error) {
+	stats, err := s.usageRepo.GetUserBreakdownStats(ctx, startTime, endTime, dim, limit)
+	if err != nil {
+		return nil, fmt.Errorf("get user breakdown stats: %w", err)
+	}
+	return stats, nil
+}
+
 func (s *DashboardService) GetBatchUserUsageStats(ctx context.Context, userIDs []int64, startTime, endTime time.Time) (map[int64]*usagestats.BatchUserUsageStats, error) {
 	stats, err := s.usageRepo.GetBatchUserUsageStats(ctx, userIDs, startTime, endTime)
 	if err != nil {