packaging: build a profile's extra venvs generically, and make audiocraft's installable

The audio build failed with 'No matching distribution found for torch==2.1.0':
audiocraft's pin is unsatisfiable on Python 3.12 at all, because torch support
for 3.12 starts at 2.2. So the venv uses the oldest torch this Python can run
and installs audiocraft --no-deps, with the deps it actually uses listed
explicitly. --no-deps is reckless in a shared venv and routine in an isolated
one: nothing in there can reach the server's stack, and the build's import check
fails the image if the version judgement turns out to be wrong.

The step is now generic — any <profile>.venv-<name>.txt builds
/opt/coderai/venvs/<name>, with an optional .nodeps beside it — rather than
hardcoding audiocraft into the Dockerfile. The next library that pins an
impossible torch will not need a new build step.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0182DhR1eFNPQyGmDrpHbedY