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/database/rpm_metadata.go

@@ -32,6 +32,11 @@ func (db *DB) InsertRPMMetadata(ctx context.Context, meta *provider.RPMMetadata)
 	return err
 }
 
+func (db *DB) DeleteRPMMetadata(ctx context.Context, repoName, filePath string) error {
+	_, err := db.Pool.Exec(ctx, `DELETE FROM rpm_metadata WHERE repo_name = $1 AND file_path = $2`, repoName, filePath)
+	return err
+}
+
 type RPMMetadataRow struct {
 	RepoName      string
 	FilePath      string