Add OCI distribution endpoints to OpenAPI/Swagger docs #4

Problem#

The Swagger UI at /swagger-ui only documents the admin API endpoints (/admin/users CRUD and permissions). All OCI Distribution Spec endpoints are missing from the OpenAPI documentation:

• GET /v2/ - Auth check
• HEAD/GET /v2/{org}/{repo}/manifests/{reference} - Manifest operations
• HEAD/GET /v2/{org}/{repo}/blobs/{digest} - Blob operations
• POST /v2/{org}/{repo}/blobs/uploads/ - Initiate upload
• PATCH/PUT /v2/{org}/{repo}/blobs/uploads/{reference} - Upload chunks
• PUT /v2/{org}/{repo}/manifests/{reference} - Push manifest
• DELETE /v2/{org}/{repo}/manifests/{reference} - Delete manifest
• DELETE /v2/{org}/{repo}/blobs/{digest} - Delete blob
• GET /v2/{org}/{repo}/tags/list - List tags

Proposed Solution#

Add utoipa annotations to all OCI endpoint handlers in blobs.rs, manifests.rs, tags.rs, and auth.rs, and include them in the OpenAPI doc generation in openapi.rs.

This would make the API self-documenting and allow users to explore and test endpoints via Swagger UI.