Report whether LLM tracing is active, without ever failing the request.

Returns the reason tracing is off when it is, so "no traces are arriving" can be diagnosed from the deployment itself rather than by reading logs.

The detailed payload (Langfuse host, environment, release, sample rate, disable reason) is operator-only. When OBSERVABILITY_STATUS_TOKEN is set, callers must present it (header X-Observability-Token; query token is accepted only in ENV=Development so tokens are not written to access logs). Only local Development may return the full payload without a token; Production and other deploy environments return a minimal liveness payload unless the token is presented (set OBSERVABILITY_STATUS_TOKEN on public staging).