Add missing asyncpg dependency for async postgres driver

config.py's DATABASE_URL uses postgresql+asyncpg but requirements.txt
only listed the sync psycopg2-binary driver, causing ModuleNotFoundError
on app startup.

backend/requirements.txt

@@ -3,6 +3,7 @@ uvicorn[standard]==0.34.0
 sqlalchemy==2.0.36
 alembic==1.14.0
 psycopg2-binary==2.9.10
+asyncpg==0.31.0
 redis==5.2.1
 pydantic==2.10.3
 pydantic-settings==2.7.0