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

pyproject.toml

@@ -24,9 +24,7 @@ dependencies = [
     "uvicorn[standard]>=0.32.0",
     "pydantic-settings>=2.6.0",
     "sqlalchemy[asyncio]>=2.0.36",
-    "asyncpg>=0.30.0",
-    "alembic>=1.14.0",
-    "pgvector>=0.3.6",
+    "aiosqlite>=0.20.0",
     "openai>=1.58.0",
     "zhipuai>=2.4.0",
     "dashscope>=1.20.0",