docs: utiliser db.engine au lieu de db.get_engine() déprécié

Co-authored-by: OpenAI/GPT-5.6-terra <vibecoder@antoineve.me>

docs/plans/2026-03-11-multi-vehicule.md

@@ -312,7 +312,7 @@ def _migrate_db(app):
     conn.close()
 
     with app.app_context():
-        engine = db.get_engine()
+        engine = db.engine
         if "motor_vehicle_id" not in columns:
             with engine.connect() as conn:
                 conn.execute(sa.text(