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

Dockerfile.postgres-with-extensions

@@ -0,0 +1,54 @@
+# PostgreSQL 15 with pgvector + pg_bigm extensions
+# 用于 AI 临床研究平台的向量检索和中文关键词检索
+#
+# 扩展版本：
+# - pgvector: 0.8.1 (向量相似度搜索)
+# - pg_bigm: 1.2 (中日韩文本全文搜索)
+#
+# 构建命令：
+#   docker build -f Dockerfile.postgres-with-extensions -t ai-clinical-postgres:v1.1 .
+#
+# 使用方法：
+#   1. 构建镜像后，修改 docker-compose.yml:
+#      image: ai-clinical-postgres:v1.1
+#   2. 重启服务：docker compose down && docker compose up -d
+#   3. 启用扩展：
+#      docker exec -it ai-clinical-postgres psql -U postgres -d ai_clinical_research -c "CREATE EXTENSION IF NOT EXISTS pg_bigm;"
+
+FROM pgvector/pgvector:pg15
+
+# 使用阿里云镜像源加速（如果在国内）
+# RUN sed -i 's/deb.debian.org/mirrors.aliyun.com/g' /etc/apt/sources.list.d/*.sources 2>/dev/null || true
+
+# 安装编译依赖
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    build-essential \
+    postgresql-server-dev-15 \
+    wget \
+    ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+# 下载并编译 pg_bigm
+# pg_bigm 是专门为中日韩（CJK）字符优化的全文搜索扩展
+RUN cd /tmp \
+    && wget -q https://github.com/pgbigm/pg_bigm/archive/refs/tags/v1.2-20200228.tar.gz \
+    && tar -xzf v1.2-20200228.tar.gz \
+    && cd pg_bigm-1.2-20200228 \
+    && make USE_PGXS=1 \
+    && make USE_PGXS=1 install \
+    && cd / \
+    && rm -rf /tmp/pg_bigm* /tmp/v1.2-20200228.tar.gz
+
+# 清理编译依赖
+RUN apt-get purge -y --auto-remove \
+    build-essential \
+    postgresql-server-dev-15 \
+    wget \
+    && rm -rf /var/lib/apt/lists/*
+
+# 添加初始化脚本（自动创建扩展）
+COPY docker-init-extensions.sql /docker-entrypoint-initdb.d/
+
+# 暴露端口
+EXPOSE 5432
+