fix: use non-deprecated sqlalchemy.orm.declarative_base import

[ayazhankadessova]

Problem

Every import of app/database.py produces this deprecation warning:

MovedIn20Warning: The ``declarative_base()`` function is now available
as sqlalchemy.orm.declarative_base(). (deprecated since: 2.0)
(Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)

This shows up in server startup, test runs, and any script that touches the database models. The project uses SQLAlchemy 2.0.48 but still imports from the old 1.x path.

Fix

One-line import change:

-from sqlalchemy.ext.declarative import declarative_base
-from sqlalchemy.orm import sessionmaker, Session
+from sqlalchemy.orm import declarative_base, sessionmaker, Session

Verification

# With -W error (turns warnings into exceptions)
python -W error -c "from app.database import Base, APIKey; print('OK')"
# OK — no warnings