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/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"`