feat(serving): add MLflow tracing to ml-serving for all agent calls

Logs one MLflow run per /recommend (params, token metrics, latency,
full prompt + tip as artifacts) and per /agents/{id}/compute and
/infer call (signals snapshot, inferred prefs, latency).

Tracing is a no-op when MLFLOW_TRACKING_URI is unset; ml-serving
starts and serves tips correctly without MLflow configured.

Refs #118 (M4: remove from production / move off critical path).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/docker/docker-compose.yml

@@ -71,6 +71,7 @@ services:
     environment:
       LITELLM_URL: ${LITELLM_URL:-http://host.docker.internal:4000}
       OLLAMA_URL: ${OLLAMA_URL:-http://host.docker.internal:11434}
+      MLFLOW_TRACKING_URI: ${MLFLOW_TRACKING_URI:-}
     extra_hosts:
       - "host.docker.internal:host-gateway"
     ports: