fix: database.py 懒初始化引擎 + list_files 返回真实 total

- database.py: engine/SessionLocal 改为延迟创建，避免 import 时副作用，
  并提供 dispose_engine() 支持配置热重载
- dao/uploaded_file_dao.py: 新增 count() 方法
- upload_service.py: list_files() 改用 dao.count() 返回数据库总条数，
  修复之前返回当前页条目数导致分页 total 语义错误的问题

app/database.py

@@ -1,4 +1,8 @@
-"""数据库引擎、Session 与 Declarative Base。"""
+"""数据库引擎、Session 与 Declarative Base。
+
+引擎采用懒初始化：首次访问 `get_engine()` 时才创建连接池，
+避免 import 时的副作用，并支持 `dispose()` 后重新加载配置。
+"""
 
 from __future__ import annotations
 
@@ -9,17 +13,41 @@ from sqlalchemy.orm import DeclarativeBase, Session, sessionmaker
 
 from .config import get_settings
 
-_settings = get_settings()
+_engine: create_engine | None = None
+_session_local: sessionmaker | None = None
 
-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)
+def get_engine() -> create_engine:
+    """返回全局 Engine 实例；首次调用时懒创建。"""
+    global _engine, _session_local
+    if _engine is None:
+        s = get_settings()
+        _engine = create_engine(
+            s.db_url(),
+            pool_pre_ping=True,
+            pool_size=s.database.pool_size,
+            pool_recycle=s.database.pool_recycle,
+            future=True,
+        )
+        _session_local = sessionmaker(
+            bind=_engine, autoflush=False, autocommit=False, future=True,
+        )
+    return _engine
+
+
+def dispose_engine() -> None:
+    """关闭连接池并清除缓存，下次访问时重新创建（配合 reload_settings 使用）。"""
+    global _engine, _session_local
+    if _engine is not None:
+        _engine.dispose()
+        _engine = None
+        _session_local = None
+
+
+def get_session_local() -> sessionmaker:
+    """返回全局 SessionLocal；确保 Engine 已初始化。"""
+    get_engine()
+    return _session_local  # type: ignore[return-value]
 
 
 class Base(DeclarativeBase):
@@ -28,7 +56,7 @@ class Base(DeclarativeBase):
 
 def get_db() -> Generator[Session, None, None]:
     """FastAPI 依赖：为每个请求产出一个 Session。"""
-    db = SessionLocal()
+    db = get_session_local()()
     try:
         yield db
     finally:
@@ -38,4 +66,5 @@ def get_db() -> Generator[Session, None, None]:
 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)
+    get_engine()
+    Base.metadata.create_all(bind=_engine)