feat(api): add admin traffic chart statistics endpoint

Add new endpoint GET /admin/logs/stats/traffic-chart to provide
aggregated traffic metrics grouped by time and model. Features include:
- Time granularity selection (hour/minute)
- Top-N model breakdown with "other" aggregation
- Metrics for request counts and token usage

Includes generated Swagger documentation.

docs/swagger/swagger.yaml

@@ -1055,6 +1055,45 @@ definitions:
       tokens:
         type: integer
     type: object
+  internal_api.TrafficBucket:
+    properties:
+      breakdown:
+        additionalProperties:
+          $ref: '#/definitions/internal_api.TrafficMetrics'
+        type: object
+      time:
+        type: string
+      timestamp:
+        type: integer
+      total:
+        $ref: '#/definitions/internal_api.TrafficMetrics'
+    type: object
+  internal_api.TrafficChartResponse:
+    properties:
+      buckets:
+        items:
+          $ref: '#/definitions/internal_api.TrafficBucket'
+        type: array
+      granularity:
+        type: string
+      models:
+        items:
+          type: string
+        type: array
+      since:
+        type: integer
+      until:
+        type: integer
+    type: object
+  internal_api.TrafficMetrics:
+    properties:
+      count:
+        type: integer
+      tokens_in:
+        type: integer
+      tokens_out:
+        type: integer
+    type: object
   internal_api.UpdateAccessRequest:
     properties:
       default_namespace:
@@ -2333,6 +2372,50 @@ paths:
       summary: Log stats (admin)
       tags:
       - admin
+  /admin/logs/stats/traffic-chart:
+    get:
+      description: Get time × model aggregated data for stacked traffic charts. Returns
+        time buckets with per-model breakdown.
+      parameters:
+      - description: 'Time granularity: hour (default) or minute'
+        enum:
+        - hour
+        - minute
+        in: query
+        name: granularity
+        type: string
+      - description: Start time (unix seconds), defaults to 24h ago
+        in: query
+        name: since
+        type: integer
+      - description: End time (unix seconds), defaults to now
+        in: query
+        name: until
+        type: integer
+      - description: Number of top models to return (1-20), defaults to 5
+        in: query
+        name: top_n
+        type: integer
+      produces:
+      - application/json
+      responses:
+        "200":
+          description: OK
+          schema:
+            $ref: '#/definitions/internal_api.TrafficChartResponse'
+        "400":
+          description: Bad Request
+          schema:
+            $ref: '#/definitions/gin.H'
+        "500":
+          description: Internal Server Error
+          schema:
+            $ref: '#/definitions/gin.H'
+      security:
+      - AdminAuth: []
+      summary: Traffic chart data (admin)
+      tags:
+      - admin
   /admin/logs/webhook:
     get:
       description: Returns current webhook notification config