feat(platform): Fix pg-boss queue conflict and add safety standards

Summary:
- Fix pg-boss queue conflict (duplicate key violation on queue_pkey)
- Add global error listener to prevent process crash
- Reduce connection pool from 10 to 4
- Add graceful shutdown handling (SIGTERM/SIGINT)
- Fix researchWorker recursive call bug in catch block
- Make screeningWorker idempotent using upsert

Security Standards (v1.1):
- Prohibit recursive retry in Worker catch blocks
- Prohibit payload bloat (only store fileKey/ID in job.data)
- Require Worker idempotency (upsert + unique constraint)
- Recommend task-specific expireInSeconds settings
- Document graceful shutdown pattern

New Features:
- PKB signed URL endpoint for document preview/download
- pg_bigm installation guide for Docker
- Dockerfile.postgres-with-extensions for pgvector + pg_bigm

Documentation:
- Update Postgres-Only async task processing guide (v1.1)
- Add troubleshooting SQL queries
- Update safety checklist

Tested: Local verification passed

backend/src/index.ts

@@ -280,3 +280,33 @@ const start = async () => {
 };
 
 start();
+
+// ============================================
+// 🛡️ 优雅关闭处理（Graceful Shutdown）
+// ============================================
+const gracefulShutdown = async (signal: string) => {
+  console.log(`\n⚠️ 收到 ${signal} 信号，开始优雅关闭...`);
+  
+  try {
+    // 1. 停止接收新请求
+    await fastify.close();
+    console.log('✅ HTTP 服务已停止');
+    
+    // 2. 停止队列（等待当前任务完成）
+    await jobQueue.stop();
+    console.log('✅ 任务队列已停止');
+    
+    // 3. 关闭数据库连接
+    await prisma.$disconnect();
+    console.log('✅ 数据库连接已关闭');
+    
+    console.log('👋 优雅关闭完成，再见！');
+    process.exit(0);
+  } catch (error) {
+    console.error('❌ 优雅关闭失败:', error);
+    process.exit(1);
+  }
+};
+
+process.on('SIGTERM', () => gracefulShutdown('SIGTERM'));
+process.on('SIGINT', () => gracefulShutdown('SIGINT'));