feat: initial project structure

- Add FastAPI app with paper browsing UI and REST API
- Add crawler service and database models
- Add scripts for DB init and manual crawl
- Add docs (api-and-ui, data-model, services)
- Add requirements and project config

scripts/init_db.py

@@ -0,0 +1,5 @@
+"""快捷脚本：初始化数据库。"""
+
+if __name__ == "__main__":
+    from app.cli import cli_app
+    cli_app(["init-db"])