Implement baseline Grafana observability and stage timing for simulation gateway and worker

[anth-volk]

Add the second observability rollout slice for policyengine-api-v2.

Scope:

• add baseline OTLP/Grafana-capable logs, traces, and metrics in shadow mode
• instrument the Modal gateway and worker with lifecycle telemetry
• emit stage-level timing for routing, worker phases, queue delay, completion, and failures
• preserve current request/response behavior while introducing the new observability path

This issue tracks the API v2 portion of the Grafana rollout plan.