Review asked for constants instead of the "cloud-proxy" and
"localai-proxy" literals. Add CloudProxyBackend and LocalAIProxyBackend next
to the other backend-name constants in pkg/model (WhisperBackend,
TransformersBackend, ...), which core/config already imports, and use them
in every production check: the proxy options builder, IsRemoteProxy,
IsCloudProxyBackendPassthrough, the PII defaults, the localai-proxy backend
hook and loader warning, and the PII middleware metadata in the routes and
the in-process MCP client.
The proxy options builder now calls IsRemoteProxy() instead of repeating
the two-backend check, so the set of proxy backends is defined once.
Assisted-by: Claude:claude-opus-5-5
Signed-off-by: Ettore Di Giacinto <mudler@localai.io>