Follow-up to #99.
## Why
Evicting or deleting a local RPM removed the \`local_files\` row but left its \`rpm_metadata\` behind. Since generated repodata is built from \`rpm_metadata\`, \`primary.xml\` kept advertising a package that no longer exists, producing 404s for clients that tried to fetch it.
## Changes
- Add \`PostDeleteHook\` and \`MetadataDeleter\` provider interfaces (symmetric to the existing \`PostUploadHook\`/\`MetadataStore\`), plus a \`DeleteRPMMetadata\` DB method.
- Implement \`AfterDelete\` in the RPM provider to drop the metadata row for the deleted file.
- Route both local delete paths — the new \`evictLocal\` and the existing files handler's \`remove\` — through a shared \`deleteLocalFile\` helper that removes the file then runs the provider's post-delete hook. Non-RPM providers have no hook, so nothing changes for them.
- Cover the cleanup with a dockerised test.
Reviewed-on: #100
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>
