refactor: 前端全面替换 any 为 unknown 并统一错误处理，后端用量记录补写请求头/体

- 前端 API 层、stores、conversation 解析器、组件全面替换 any 为 unknown/具体类型
- 错误处理统一使用 parseApiError/getErrorStatus 替代 err.response?.data?.detail 模式
- 后端 handler/TaskService/UsageLifecycle/StreamTracker 链路传递 request_headers/request_body
- streaming/pending 状态更新时可补写客户端和提供商的请求头及请求体
- 新增 TaskService 和 UsageService 相关测试

frontend/src/views/admin/UserStats.vue

@@ -166,11 +166,23 @@ const compareUserId = ref<string>('__none__')
 const leaderboard = ref<LeaderboardItem[]>([])
 const leaderboardLoading = ref(false)
 
-const userSummary = ref<any | null>(null)
+interface UsageSummary {
+  total_requests: number
+  total_tokens: number
+  total_cost: number
+  error_rate: number
+}
+
+interface TimeSeriesItem {
+  date: string
+  total_cost: number
+}
+
+const userSummary = ref<UsageSummary | null>(null)
 const summaryLoading = ref(false)
 
-const series = ref<any[]>([])
-const comparisonSeries = ref<any[]>([])
+const series = ref<TimeSeriesItem[]>([])
+const comparisonSeries = ref<TimeSeriesItem[]>([])
 const seriesLoading = ref(false)
 
 function buildTimeRangeParams() {