fix(deps): switch psycopg2 to psycopg2-binary (#5937)

Building psycopg2 from source needs libpq-dev/pg_config, which isn't
in the Docker image or most dev hosts, so pip install silently fails
and Postgres users hit ModuleNotFoundError at import time.

requirements.txt

@@ -51,3 +51,8 @@ pytest-asyncio
 # TestClient import when only classic httpx is present. Runtime code keeps
 # using `httpx` above; this is test-client only.
 httpx2
+# DATABASE_URL defaults to sqlite (core/database.py), but when pointed at an
+# external Postgres, SQLAlchemy's postgresql dialect imports psycopg2 inside
+# create_engine() and raises ModuleNotFoundError if missing. -binary avoids
+# needing libpq-dev/pg_config on the host/image to compile it.
+psycopg2-binary