fix: prune RPM metadata when a local file is evicted (#100)

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>

internal/provider/rpm/rpm.go

@@ -151,6 +151,15 @@ func (p *Provider) AfterUpload(ctx context.Context, repoName, storagePath, conte
 	slog.Info("rpm metadata: parsed", "repo", repoName, "name", meta.Name, "version", meta.Version, "arch", meta.Arch)
 }
 
+func (p *Provider) AfterDelete(ctx context.Context, repoName, storagePath string, db provider.MetadataDeleter) error {
+	if err := db.DeleteRPMMetadata(ctx, repoName, storagePath); err != nil {
+		slog.Error("rpm metadata: delete failed", "repo", repoName, "path", storagePath, "error", err)
+		return err
+	}
+	slog.Info("rpm metadata: deleted", "repo", repoName, "path", storagePath)
+	return nil
+}
+
 func rpmDepFromEntry(e rpmlib.Dependency) provider.RPMDep {
 	dep := provider.RPMDep{Name: e.Name()}
 	if e.Flags() != 0 {