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

app/database.py

@@ -0,0 +1,41 @@
+"""数据库引擎、会话工厂、初始化。"""
+
+from sqlalchemy import event, create_engine
+from sqlalchemy.orm import DeclarativeBase, sessionmaker
+
+from app.config import settings
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def _make_engine():
+    """创建 SQLite 引擎，启用 foreign_keys。"""
+    engine = create_engine(
+        settings.DATABASE_URL,
+        echo=settings.APP_DEBUG,
+        connect_args={"check_same_thread": False},
+    )
+
+    @event.listens_for(engine, "connect")
+    def _set_sqlite_pragma(dbapi_connection, _connection_record):
+        cursor = dbapi_connection.cursor()
+        cursor.execute("PRAGMA foreign_keys=ON")
+        cursor.execute("PRAGMA journal_mode=WAL")
+        cursor.close()
+
+    return engine
+
+
+engine = _make_engine()
+SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False)
+
+
+def get_db():
+    """FastAPI 依赖注入：获取数据库会话。"""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()