Implement /v2/_catalog endpoint for repository listing #5

Problem#

The OCI Distribution Spec defines a GET /v2/_catalog endpoint that lists all repositories in the registry. This endpoint is not implemented in grain. Tools like crane catalog, skopeo, and some CI systems expect this endpoint to exist.

Proposed Solution#

• Add a GET /v2/_catalog route
• Scan the ./tmp/manifests directory structure to enumerate {org}/{repo} pairs
• Require authentication (at minimum read permission)
• Support pagination via n and last query parameters per the spec
• Return {"repositories": ["org/repo1", "org/repo2"]} format

Considerations#

• Should respect per-user permissions (only list repos the authenticated user can read)
• Could be expensive on large registries; consider caching