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/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 := `