refactor: 移除 PostgreSQL 支持，简化为纯 SQLite 部署

- config.py: DATABASE_URL 默认值改为 SQLite
- database.py: 移除 PostgreSQL 分支，简化为纯 SQLite
- models/base.py: 移除 pgvector 导入和条件分支
- search_service.py: 移除 _search_postgres 方法
- import_service.py: 移除 pgvector 相关代码
- requirements.txt: 移除 asyncpg/alembic/pgvector 依赖
- pyproject.toml: 同步移除相关依赖
- docker-compose.yml: 移除 db 服务，- 删除 alembic.ini/Dockerfile.db/sql 目录
- README.md: 更新文档，移除 PostgreSQL 相关内容

适合 NAS 等资源受限环境的轻量级部署

docker-compose.yml

@@ -1,7 +1,6 @@
 version: "3.8"
 
 services:
-  # ── 应用服务 ──
   app:
     build:
       context: .
@@ -11,45 +10,12 @@ services:
     ports:
       - "${APP_PORT:-8000}:8000"
     volumes:
-      - ./data:/app/data          # 数据目录挂载
+      - ./data:/app/data
     env_file:
       - .env
-    depends_on:
-      db:
-        condition: service_healthy
-    environment:
-      - DATABASE_URL=postgresql+asyncpg://postgres:${POSTGRES_PASSWORD:-postgres}@db:5432/edu_brain
     healthcheck:
       test: ["CMD", "python", "-c", "import urllib.request; urllib.request.urlopen('http://localhost:8000/health')"]
       interval: 30s
       timeout: 10s
       retries: 3
       start_period: 60s
-
-  # ── PostgreSQL 数据库（pgvector + zhparser 自定义镜像） ──
-  db:
-    build:
-      context: .
-      dockerfile: Dockerfile.db
-    container_name: edu-brain-db
-    restart: unless-stopped
-    ports:
-      - "${DB_PORT:-5432}:5432"
-    volumes:
-      - postgres_data:/var/lib/postgresql/data
-      - ./sql/init.sql:/docker-entrypoint-initdb.d/01-init.sql:ro
-    environment:
-      POSTGRES_USER: postgres
-      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD:-postgres}
-      POSTGRES_DB: edu_brain
-      POSTGRES_HOST_AUTH_METHOD: scram-sha-256
-    healthcheck:
-      test: ["CMD-SHELL", "pg_isready -U postgres -d edu_brain"]
-      interval: 10s
-      timeout: 5s
-      retries: 5
-      start_period: 30s
-
-volumes:
-  postgres_data:
-    driver: local