build: install only libpq5 in the Docker image

[lesnik512]

The apt block installed build-essential and libpq-dev "for psycopg2", but the project depends on psycopg (3). Every locked dependency ships a Linux wheel for Python 3.14, so nothing is compiled. Pure-Python psycopg only needs the libpq runtime library, which Alembic uses inside the container.

Replaced both packages with libpq5.

Verification:

• With the apt block removed entirely, just test fails at alembic downgrade base with ImportError: no pq wrapper available ... libpq library not found.
• With libpq5, just build and just test pass: 31 tests, 100% coverage.
• Image size in the litestar template: 692MB after the change, vs 1.2GB for an older local build with the old apt block (approximate comparison).