fix: prune RPM metadata when a local file is evicted

Evicting or deleting a local RPM removed the local_files row but left its
rpm_metadata behind, so generated repodata kept listing a package that no
longer exists. Deletes now run a provider cleanup hook symmetric to the
existing upload hook.

- add PostDeleteHook and MetadataDeleter provider interfaces, plus a
  DeleteRPMMetadata DB method
- implement AfterDelete in the RPM provider to drop the metadata row
- route both local delete paths (evictLocal and the files handler) through
  a shared deleteLocalFile helper that removes the file then runs the hook
- cover the cleanup with a dockerised test

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