The OCI handler had /v2/ baked into all regex patterns and Location headers.
When basePath was set to /v2 (as in stack.gallery), stripping it removed the
prefix that patterns expected, causing all OCI endpoints to 404.
Now patterns match on bare paths after basePath stripping, working correctly
regardless of the basePath value.
Also adds configurable apiPrefix to OCI upstream class (default /v2) for
registries behind reverse proxies with custom path prefixes.
