Observability: optional OpenObserve/OTLP support via config

[lost-rob0t]

Coordinate with lost-rob0t/starintel-server#128.

Add optional operator-controlled OTLP observability to the StarIntel messaging broker layer.

• Optional init/config observability block; default OFF.
• Missing config means no exporter/network traffic and no OpenObserve/collector requirement.
• Configure endpoint/protocol, service/environment/stream, signal toggles, bounded queue/timeouts, and auth via env/secret references.
• Instrument router startup, connection lifecycle, send/receive/route outcomes, queue/backpressure state, retries, disconnects, and errors.
• Use safe message metadata only; never export message bodies, target data, or credentials by default.
• Exporter failure must not block or change routing semantics.
• Tests cover disabled zero-network behavior, configured export, and exporter outage.
• Document generic OTLP and OpenObserve examples.

Docs: https://openobserve.ai/docs/ingestion/

Operator observability only; no product telemetry.