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

docker-compose.yml

@@ -1,9 +1,9 @@
 version: '3.8'
 
 services:
-  # PostgreSQL 数据库
+  # PostgreSQL 数据库 (with pgvector extension)
   postgres:
-    image: postgres:15-alpine
+    image: pgvector/pgvector:pg15
     container_name: ai-clinical-postgres
     environment:
       POSTGRES_DB: ai_clinical_research