refactor: update registry type from 'docker-hub' to 'oci' across seed data (#325)

Updates the registry type for all Docker Hub packages in the seed data
from `docker-hub` to `oci` to align with the standardized OCI registry
format. This change affects the `registry_type` field and simplifies the
`identifier` field by removing the full Docker Hub URL prefix.

Changes:
- Updated `registry_type` from `docker-hub` to `oci` for all Docker
packages
- Simplified `identifier` field to use just the repository name/path
instead of full URLs
- Maintained `registry_base_url` as `https://docker.io` for all entries

This refactoring improves consistency and aligns with OCI standards
while maintaining backward compatibility through the registry base URL.

docs/server-registry-api/openapi.yaml

@@ -278,11 +278,11 @@ components:
       properties:
         registry_type:
           type: string
-          description: Registry type indicating how to download packages (e.g., 'npm', 'pypi', 'docker-hub', 'nuget', 'mcpb')
+          description: Registry type indicating how to download packages (e.g., 'npm', 'pypi', 'oci', 'nuget', 'mcpb')
           examples:
             - "npm"
             - "pypi"
-            - "docker-hub"
+            - "oci"
             - "nuget"
             - "mcpb"
         registry_base_url: