Fix Cloud crashloop by downloading model files as runtime user

[mattvick]

What changed

• Moved the download-files step out of the Docker build stage and into the runtime stage of Dockerfile.
• Removed build-stage RUN pnpm download-files.
• Added RUN node dist/main.js download-files after USER appuser.

Why

Resolves #30

The agent was crashing in LiveKit Cloud because turn-detector model files were downloaded under the root user cache (/root/.cache/...) during build, but the worker runs as appuser and looks in /app/.cache/... at runtime.
This mismatch caused missing model files (model_q8.onnx) and startup failure (CrashLoop).

Result

• Model artifacts are now cached under the same user/home directory used at runtime.
• Agent startup no longer fails due to missing turn-detector files in Cloud deployments.

Validation

• Deployment logs previously showed missing cache/model errors and runner initialization timeout.
• With this change, runtime download path and runtime user are aligned, which resolves the cache-location mismatch causing the crash.