Replace db.Model with DeclarativeBase from SQLAlchemy for Flask-SQLAlchemy-Lite migration

docs/source/examples.rst

@@ -13,7 +13,7 @@ The following is an example configuration for *Mia! Accounting*.
 
     from flask import Response, redirect
     from .auth import current_user()
-    from .modules import User
+    from .modules import Base, User
 
     def create_app(test_config=None) -> Flask:
         app: Flask = Flask(__name__)
@@ -36,6 +36,10 @@ The following is an example configuration for *Mia! Accounting*.
             def unauthorized(self) -> Response:
                 return redirect("/login")
 
+            @property
+            def base(self) -> type[DeclarativeBase]:
+                return Base
+
             @property
             def cls(self) -> type[User]:
                 return User