feat: add BUCKET_PREFIX support for S3 uploads

[jpswelch]

Motivation

Add support for BUCKET_PREFIX environment variable to allow organizing S3 uploaded files under a custom prefix/folder path.

• When BUCKET_PREFIX is set: uses upload_file_to_bucket with the prefix (e.g., outputs/comfyui/{job_id}/filename.png)
• When BUCKET_PREFIX is not set: uses original upload_image for backward compatibility

Issues closed

[TimPietruskyRunPod]

Reviewed alongside the broader bug-triage pass on the repo today. The change itself looks good:

• Opt-in via env var, fully backward compatible (else branch keeps the existing upload_image call) ✅
• .strip("/") handles both leading/trailing slashes ✅
• Documentation update is clear and includes the path format example ✅

One small thing before merge: this repo uses Changesets for release notes (see .changeset/). Could you add a file like .changeset/feat-add-bucket-prefix.md:

---
"worker-comfyui": minor
---

feat: add `BUCKET_PREFIX` env var to organize S3 uploaded files under a custom prefix path. When set, uses `rp_upload.upload_file_to_bucket` and the path becomes `{prefix}/{job_id}/{original_filename}`. When unset, behavior is unchanged.

That way the release workflow picks it up and ships it as a minor bump. Once that's added I'll be happy to merge.

Also FYI — once #223 lands, this PR will get an automatically-built base image tagged worker-comfyui-191:base (or similar) so we can deploy a test endpoint and confirm the upload paths actually work against R2/GCS before merging.