Remove keycloak from llm-proxy and put it behind the auth-proxy

[fabianvf]

llm-proxy currently embeds its own keycloak-based authentication. With the
new auth model, all auth is handled by the auth-proxy that already fronts
the rest of the stack. llm-proxy should sit fully behind the auth-proxy
and stop doing its own auth.

What needs to happen

• Strip the keycloak integration out of the operator-managed llm-proxy
  deployment (env vars, mounts, sidecars, anything related).
• Reconfigure llm-proxy to be reached only through the auth-proxy so that
  requests arrive already-authenticated.
• Clean up any operator-side configuration knobs that only existed to
  feed keycloak credentials.
• Update generated manifests / CR docs to reflect the new topology.

Relies on #560

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