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/jobs/ocrJob.js

@@ -249,28 +249,39 @@ async function processClaimedInvoice(prisma, config, claimedInvoice, workerId, l
         },
         "OCR invoice marked as duplicate",
       );
-      return;
+      return { outcome: "duplicate" };
     }
 
     await markInvoiceAsAvailable(prisma, claimedInvoice.id, normalized, ocrResultId);
+    return { outcome: "available" };
   } catch (error) {
     const message = trimErrorMessage(error);
     await markInvoiceAsFailed(prisma, claimedInvoice, message, config);
     logger.error({ err: error, invoiceId: claimedInvoice.id, workerId }, "OCR processing failed");
+    return { outcome: "failed" };
   }
 }
 
 async function runOcrOnce(prisma, config, logger, workerId = `ocr:${process.pid}`) {
   const claimed = await claimPendingInvoices(prisma, config, workerId);
   if (!claimed.length) {
-    return { claimed: 0 };
+    return { claimed: 0, available: 0, duplicate: 0, failed: 0 };
   }
 
+  const stats = { claimed: claimed.length, available: 0, duplicate: 0, failed: 0 };
+
   for (const item of claimed) {
-    await processClaimedInvoice(prisma, config, item, workerId, logger);
+    const result = await processClaimedInvoice(prisma, config, item, workerId, logger);
+    if (result?.outcome === "available") {
+      stats.available += 1;
+    } else if (result?.outcome === "duplicate") {
+      stats.duplicate += 1;
+    } else if (result?.outcome === "failed") {
+      stats.failed += 1;
+    }
   }
 
-  return { claimed: claimed.length };
+  return stats;
 }
 
 function startOcrJob(fastify) {
@@ -289,7 +300,16 @@ function startOcrJob(fastify) {
 
     running = true;
     try {
-      await runOcrOnce(fastify.prisma, fastify.config, fastify.log, workerId);
+      const result = await runOcrOnce(fastify.prisma, fastify.config, fastify.log, workerId);
+      if (result.claimed > 0) {
+        fastify.log.info(
+          {
+            workerId,
+            ...result,
+          },
+          "OCR job tick processed",
+        );
+      }
     } catch (error) {
       fastify.log.error({ err: error }, "OCR job tick failed");
     } finally {