runpod: resolve a venv with uv when pip cannot

pip spent an hour and gigabytes of RAM on sglang-kt's forty exact pins
without installing a byte; uv resolved the same set in seconds. A
<profile>.venv-<name>.uv marker opts a venv in. Also drop __pycache__
from baked venvs.

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