数据链路 P1 修复：ETag 缓存 + 校准漂移检测

P1-5 ETag 支持：
- 后端新增 _etag_middleware：GET /api/* 自动返回 ETag (MD5)
- 支持 If-None-Match 请求头，匹配时返回 304 + 30s Cache-Control
- 前端 cache: no-store → default，浏览器自动处理 ETag/304 节省带宽

P1-6 校准漂移检测：
- probability_calibration.py 新增 check_calibration_drift()
- 对比最近 200 条 daily_records 的 CRPS 与校准基线
- 漂移 >15% 时返回 warning 提示重新训练
- 集成到 /api/system/status 的 probability.drift 字段

Tested: python -m ruff check ., npx tsc --noEmit

frontend/lib/dashboard-client.ts

@@ -68,7 +68,7 @@ async function fetchJson<T>(url: string, options?: { timeoutMs?: number }): Prom
   try {
     response = await fetchBackendApi(url, {
       headers,
-      cache: "no-store",
+      cache: "default",
       signal: controller?.signal,
     });
   } catch (error) {
@@ -238,7 +238,7 @@ export const dashboardClient = {
     void fetch(`/api/system/priority-warm?${params.toString()}`, {
       method: "POST",
       headers: { Accept: "application/json" },
-      cache: "no-store",
+      cache: "default",
       keepalive: true,
     }).catch(() => {});
   },
@@ -418,7 +418,7 @@ export const dashboardClient = {
     }).then((headers) => fetchBackendApi("/api/scan/terminal/ai", {
       method: "POST",
       headers,
-      cache: "no-store",
+      cache: "default",
       body: JSON.stringify({
         filters: payload.filters,
         snapshot_id: snapshotId || null,