Implement baseline Grafana observability and stage timing for legacy simulation submission

[anth-volk]

Add the second observability rollout slice for policyengine-api.

Scope:

• add baseline OTLP/Grafana-capable logs, traces, and metrics in shadow mode
• instrument the legacy economy submission path and Modal client with coarse lifecycle telemetry
• emit stage-level timing for request setup, submission, and related failure handling
• preserve existing behavior while introducing the new observability path

This issue tracks the API v1 portion of the Grafana rollout plan.