init: 从 /root/zikai 根目录迁入

把原本散落在 /root/zikai 的 FastAPI 服务整理到 server/ 子目录，
开启独立 git 与 venv。运行时数据（config.yaml / keys / uploads）
按 .gitignore 留在工作目录但不入仓。

app/database.py

@@ -0,0 +1,41 @@
+"""数据库引擎、Session 与 Declarative Base。"""
+
+from __future__ import annotations
+
+from collections.abc import Generator
+
+from sqlalchemy import create_engine
+from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
+
+from .config import get_settings
+
+_settings = get_settings()
+
+engine = create_engine(
+    _settings.db_url(),
+    pool_pre_ping=True,
+    pool_size=_settings.database.pool_size,
+    pool_recycle=_settings.database.pool_recycle,
+    future=True,
+)
+
+SessionLocal = sessionmaker(bind=engine, autoflush=False, autocommit=False, future=True)
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def get_db() -> Generator[Session, None, None]:
+    """FastAPI 依赖：为每个请求产出一个 Session。"""
+    db = SessionLocal()
+    try:
+        yield db
+    finally:
+        db.close()
+
+
+def init_db_schema() -> None:
+    """按需建表（幂等）。先导入 models 以注册映射。"""
+    from . import models  # noqa: F401
+    Base.metadata.create_all(bind=engine)