feat: add local RPM repository with on-demand repodata (#53)

## Summary
- Upload RPMs to local repos, metadata parsed async via cavaliergopher/rpm
- Repodata (repomd.xml, primary/filelists/other.xml.gz) generated on-demand from DB — nothing stored in S3
- RPM provider implements LocalUploader, PostUploadHook, and LocalIndexer
- New rpm_metadata table for parsed RPM header data (name, version, deps, etc.)
- New provider interfaces: PostUploadHook, BlobReader, MetadataStore, RPMMetadataReader

## Test plan
- [x] Upload cowsay RPM from epel → async metadata parse confirmed in logs
- [x] repomd.xml generated with correct hashes → primary.xml.gz has correct metadata
- [x] `dnf install` from local repo: download + install successful
- [x] Bad file rejection (.txt → 400), overwrite rejection (409)

Reviewed-on: #53
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

go.sum

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+github.com/cavaliergopher/rpm v1.3.0 h1:UHX46sasX8MesUXXQ+UbkFLUX4eUWTlEcX8jcnRBIgI=
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