refactor(api): update traffic chart response structure

Change the traffic chart API response from bucket-based to series-based
to better support frontend visualization libraries. The new format
provides a shared X-axis and aligned data arrays for each model series.

- Replace `buckets` with `x` and `series` in response
- Implement data alignment and zero-filling for time slots
- Update Swagger documentation including pending definitions

BREAKING CHANGE: The `GET /admin/logs/stats/traffic-chart` response
schema has changed. `buckets` and `models` fields are removed.

docs/api.md

@@ -371,6 +371,49 @@ curl "http://localhost:8080/admin/logs/stats?group_by=minute&since=1704150000&un
 }
 ```
 
+#### 流量图表统计
+`GET /admin/logs/stats/traffic-chart`
+
+**参数**：
+| 参数 | 类型 | 说明 | 约束 |
+| :--- | :--- | :--- | :--- |
+| `granularity` | string | 时间粒度：`hour` / `minute` | `minute` 必须提供 `since` + `until` 且跨度 ≤ 6 小时 |
+| `since` | int | 起始时间 (Unix 秒) | 默认 24 小时前 |
+| `until` | int | 结束时间 (Unix 秒) | 默认当前 |
+| `top_n` | int | Top 模型数量 | 1-20，默认 5，其余聚合为 `other` |
+
+**响应示例**：
+```json
+{
+  "granularity": "hour",
+  "since": 1735689600,
+  "until": 1735776000,
+  "x": {
+    "labels": ["2025-01-01T00:00:00Z", "2025-01-01T01:00:00Z"],
+    "timestamps": [1735689600, 1735693200],
+    "totals": {
+      "data": [2050, 1800],
+      "tokens_in": [82000, 70000],
+      "tokens_out": [128000, 110000]
+    }
+  },
+  "series": [
+    {
+      "name": "gpt-4-turbo",
+      "data": [1200, 900],
+      "tokens_in": [50000, 38000],
+      "tokens_out": [80000, 62000]
+    },
+    {
+      "name": "other",
+      "data": [850, 900],
+      "tokens_in": [32000, 32000],
+      "tokens_out": [48000, 48000]
+    }
+  ]
+}
+```
+
 ### 6.2 API Key 状态筛选
 
 `GET /admin/api-keys?status=active|suspended|auto_disabled|manual_disabled`