Manifest delete (DELETE /v2/<name>/manifests/<digest>): rejects tag
references with 405 UNSUPPORTED per OCI spec, cascades to tags and
manifest_blobs via ON DELETE CASCADE, returns 202 Accepted.
Blob delete (DELETE /v2/<name>/blobs/<digest>): removes manifest_blobs
associations only — blob row and file are preserved for GC to handle,
since other repos may reference the same content-addressed blob.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>