feat(pkb): Integrate pgvector and create Dify replacement plan

Summary:
- Migrate PostgreSQL to pgvector/pgvector:pg15 Docker image
- Successfully install and verify pgvector 0.8.1 extension
- Create comprehensive Dify-to-pgvector migration plan
- Update PKB module documentation with pgvector status
- Update system documentation with pgvector integration

Key changes:
- docker-compose.yml: Switch to pgvector/pgvector:pg15 image
- Add EkbDocument and EkbChunk data model design
- Design R-C-R-G hybrid retrieval architecture
- Add clinical data JSONB fields (pico, studyDesign, regimen, safety, criteria, endpoints)
- Create detailed 10-day implementation roadmap

Documentation updates:
- PKB module status: pgvector RAG infrastructure ready
- System status: pgvector 0.8.1 integrated
- New: Dify replacement development plan (01-Dify替换为pgvector开发计划.md)
- New: Enterprise medical knowledge base solution V2

Tested: PostgreSQL with pgvector verified, frontend and backend functionality confirmed

Dockerfile.postgres-pgvector

@@ -0,0 +1,23 @@
+# 基于已有的 postgres:15-alpine 镜像构建 pgvector 版本
+# pgvector 版本：0.8.0（与阿里云 RDS PostgreSQL 15 保持一致）
+FROM postgres:15-alpine
+
+# 使用阿里云镜像源加速 Alpine 包下载
+RUN sed -i 's/dl-cdn.alpinelinux.org/mirrors.aliyun.com/g' /etc/apk/repositories
+
+# 复制本地下载的 pgvector 源码包
+COPY pgvector-0.8.0.tar.gz /tmp/pgvector-0.8.0.tar.gz
+
+# 安装编译依赖并编译 pgvector（禁用 LLVM bitcode 编译）
+RUN apk add --no-cache --virtual .build-deps \
+        build-base \
+    && cd /tmp \
+    && tar -xzf pgvector-0.8.0.tar.gz \
+    && cd pgvector-0.8.0 \
+    && sed -i 's/^with_llvm.*/with_llvm = no/' /usr/local/lib/postgresql/pgxs/src/Makefile.global \
+    && make USE_PGXS=1 PG_CONFIG=/usr/local/bin/pg_config \
+    && make USE_PGXS=1 PG_CONFIG=/usr/local/bin/pg_config install \
+    && cd / \
+    && rm -rf /tmp/pgvector-0.8.0 /tmp/pgvector-0.8.0.tar.gz \
+    && apk del .build-deps
+