feat(engine): 实现同步引擎与 engine_status 快照

apps/engine 独立进程（syncRound + 防重叠调度）；上游 log/token；QPU 采样；
server 读 engine_status；时刻出参 toUnixSec；中文日志；zod-openapi 扩展顺序修复。

apps/engine/src/loop.ts

@@ -0,0 +1,75 @@
+/**
+ * 调度循环（方案 A）：一轮结束 → setTimeout 下一枪；inFlight 防重叠。
+ * 启停不得抹掉已有 last_round。
+ */
+import { appLogger } from "@new-api-key-usage/core";
+import { syncRound, type SyncRoundDeps } from "./sync-round.ts";
+
+const log = appLogger("engine");
+
+export interface LoopOptions extends SyncRoundDeps {
+  intervalMs: number;
+}
+
+export function createSyncLoop(opts: LoopOptions) {
+  let inFlight = false;
+  let stopped = false;
+  let timer: ReturnType<typeof setTimeout> | null = null;
+  let last_round: unknown = null;
+
+  async function persist(running: boolean) {
+    await opts.repos.engine.upsertSnapshot({
+      running,
+      sync_interval_ms: opts.intervalMs,
+      last_round,
+    });
+  }
+
+  async function runOnce(): Promise<void> {
+    if (stopped) return;
+    if (inFlight) {
+      log.warn("上一轮尚未结束，跳过本枪");
+      schedule();
+      return;
+    }
+    inFlight = true;
+    try {
+      last_round = await syncRound(opts);
+      await persist(!stopped);
+    } catch (e) {
+      log.error(`同步轮次异常：${e}`);
+      await persist(!stopped).catch(() => {});
+    } finally {
+      inFlight = false;
+      schedule();
+    }
+  }
+
+  function schedule() {
+    if (stopped) return;
+    timer = setTimeout(() => {
+      void runOnce();
+    }, opts.intervalMs);
+  }
+
+  return {
+    start() {
+      log.info(`同步引擎启动：周期=${opts.intervalMs}ms`);
+      void (async () => {
+        const snap = await opts.repos.engine.getSnapshot();
+        last_round = snap?.last_round ?? null;
+        await persist(true).catch(() => {});
+        void runOnce();
+      })();
+    },
+    async stop() {
+      stopped = true;
+      if (timer) clearTimeout(timer);
+      // 等在跑的一轮结束，避免 stop 与 upsert 交错
+      for (let i = 0; i < 50 && inFlight; i++) {
+        await new Promise((r) => setTimeout(r, 100));
+      }
+      await persist(false).catch(() => {});
+    },
+  };
+}