Fix Base import in app/db/base.py for Alembic migrations

The error was occurring because migrations/env.py was trying to import Base from app.db.base, but it wasn't being exported there. This change adds the proper import and re-export of Base in app.db.base.py, fixing the database migration error.

app/db/base.py

@@ -1 +1,5 @@
+from app.db.base_class import Base
+from app.models.user import User
+from app.models.client import Client
+from app.models.invoice import Invoice, InvoiceItem
 # Import all models here for Alembic to detect them