feat(server): readiness 与作业统计可观测性（E11）

- 新增 /ready（DB 探活失败返回 503）
- OCR 作业输出 claimed/available/duplicate/failed 统计
- README 补充 /health 与 /ready 说明

server/src/app.js

@@ -148,6 +148,21 @@ async function buildApp() {
     return reply.send({ requestId: request.id, data: { ok: true } });
   });
 
+  app.get("/ready", async (request, reply) => {
+    try {
+      await app.prisma.$queryRaw`SELECT 1`;
+      return reply.send({ requestId: request.id, data: { ok: true } });
+    } catch (error) {
+      app.log.error({ err: error, requestId: request.id }, "Readiness check failed");
+      throw new AppError({
+        code: "SYS_503",
+        message: "Service Unavailable",
+        statusCode: 503,
+        details: { db: "unreachable" },
+      });
+    }
+  });
+
   await app.register(authRoutes, { prefix: "/api/auth" });
   await app.register(meRoutes, { prefix: "/api" });
   await app.register(reimbursementRoutes, { prefix: "/api" });