feat(dashboard): add group usage distribution chart to usage page

Add a doughnut chart showing usage statistics broken down by group on
the admin usage records page. The chart appears alongside the existing
model distribution chart (2-column grid), with the token usage trend
chart moved to a separate full-width row below.

Changes:
- backend/pkg/usagestats: add GroupStat type
- backend/service: add GetGroupStatsWithFilters interface method and implementation
- backend/repository: implement GetGroupStatsWithFilters with LEFT JOIN groups
- backend/handler: add GetGroupStats handler with full filter support
- backend/routes: register GET /admin/dashboard/groups route
- backend/tests: add GetGroupStatsWithFilters stubs to contract/sora tests
- frontend/types: add GroupStat interface
- frontend/api: add getGroupStats API function and types
- frontend/components: add GroupDistributionChart.vue doughnut chart
- frontend/views: update UsageView layout and load group stats in parallel
- frontend/i18n: add groupDistribution, group, noGroup keys (zh + en)

backend/internal/handler/admin/dashboard_handler.go

@@ -333,6 +333,76 @@ func (h *DashboardHandler) GetModelStats(c *gin.Context) {
 	})
 }
 
+// GetGroupStats handles getting group usage statistics
+// GET /api/v1/admin/dashboard/groups
+// Query params: start_date, end_date (YYYY-MM-DD), user_id, api_key_id, account_id, group_id, request_type, stream, billing_type
+func (h *DashboardHandler) GetGroupStats(c *gin.Context) {
+	startTime, endTime := parseTimeRange(c)
+
+	var userID, apiKeyID, accountID, groupID int64
+	var requestType *int16
+	var stream *bool
+	var billingType *int8
+
+	if userIDStr := c.Query("user_id"); userIDStr != "" {
+		if id, err := strconv.ParseInt(userIDStr, 10, 64); err == nil {
+			userID = id
+		}
+	}
+	if apiKeyIDStr := c.Query("api_key_id"); apiKeyIDStr != "" {
+		if id, err := strconv.ParseInt(apiKeyIDStr, 10, 64); err == nil {
+			apiKeyID = id
+		}
+	}
+	if accountIDStr := c.Query("account_id"); accountIDStr != "" {
+		if id, err := strconv.ParseInt(accountIDStr, 10, 64); err == nil {
+			accountID = id
+		}
+	}
+	if groupIDStr := c.Query("group_id"); groupIDStr != "" {
+		if id, err := strconv.ParseInt(groupIDStr, 10, 64); err == nil {
+			groupID = id
+		}
+	}
+	if requestTypeStr := strings.TrimSpace(c.Query("request_type")); requestTypeStr != "" {
+		parsed, err := service.ParseUsageRequestType(requestTypeStr)
+		if err != nil {
+			response.BadRequest(c, err.Error())
+			return
+		}
+		value := int16(parsed)
+		requestType = &value
+	} else if streamStr := c.Query("stream"); streamStr != "" {
+		if streamVal, err := strconv.ParseBool(streamStr); err == nil {
+			stream = &streamVal
+		} else {
+			response.BadRequest(c, "Invalid stream value, use true or false")
+			return
+		}
+	}
+	if billingTypeStr := c.Query("billing_type"); billingTypeStr != "" {
+		if v, err := strconv.ParseInt(billingTypeStr, 10, 8); err == nil {
+			bt := int8(v)
+			billingType = &bt
+		} else {
+			response.BadRequest(c, "Invalid billing_type")
+			return
+		}
+	}
+
+	stats, err := h.dashboardService.GetGroupStatsWithFilters(c.Request.Context(), startTime, endTime, userID, apiKeyID, accountID, groupID, requestType, stream, billingType)
+	if err != nil {
+		response.Error(c, 500, "Failed to get group statistics")
+		return
+	}
+
+	response.Success(c, gin.H{
+		"groups":     stats,
+		"start_date": startTime.Format("2006-01-02"),
+		"end_date":   endTime.Add(-24 * time.Hour).Format("2006-01-02"),
+	})
+}
+
 // GetAPIKeyUsageTrend handles getting API key usage trend data
 // GET /api/v1/admin/dashboard/api-keys-trend
 // Query params: start_date, end_date (YYYY-MM-DD), granularity (day/hour), limit (default 5)

backend/internal/handler/sora_gateway_handler_test.go

@@ -320,6 +320,9 @@ func (s *stubUsageLogRepo) GetUsageTrendWithFilters(ctx context.Context, startTi
 func (s *stubUsageLogRepo) 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 *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) GetAPIKeyUsageTrend(ctx context.Context, startTime, endTime time.Time, granularity string, limit int) ([]usagestats.APIKeyUsageTrendPoint, error) {
 	return nil, nil
 }