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/scripts/run-ocr-once.js

@@ -15,7 +15,9 @@ const { runOcrOnce } = require("../jobs/ocrJob");
     }
 
     const result = await runOcrOnce(prisma, config, console, `ocr-script:${process.pid}`);
-    process.stdout.write(`claimed=${result.claimed}\n`);
+    process.stdout.write(
+      `claimed=${result.claimed} available=${result.available} duplicate=${result.duplicate} failed=${result.failed}\n`,
+    );
   } finally {
     await prisma.$disconnect();
     await pool.end();