Decide OTLP collector/export destination for UK Chat observability

Context

PR #118 intentionally instruments UK Chat with policyengine-observability without configuring an external OTLP trace/metric destination. Structured observability logs currently go through the backend's normal stdout/logging path and are captured by Modal runtime logs.

Decision to make

Choose the platform-level OTLP export path for UK Chat observability data, including:

• Whether UK Chat should export OTel traces/metrics directly or through a shared collector.
• The target destination for traces and metrics.
• The environment/secrets needed for OTEL_EXPORTER_OTLP_ENDPOINT and related auth/configuration.
• Whether this should be configured per service or through a shared PolicyEngine observability deployment pattern.

Acceptance criteria

• Document the selected OTLP destination and collector strategy.
• Define the required environment variables/secrets for Modal deployments.
• Confirm how this fits with the broader PolicyEngine observability approach across services.
• Update UK Chat deployment configuration only after the destination decision is made.

Related: #118

Start by reading PR #118 and the existing UK Chat Modal deployment configuration, then compare it with the broader PolicyEngine observability approach. Determine the collector strategy, OTLP destination, and required Modal environment variables or secrets; done means the decision is documented, its cross-service fit is confirmed, and deployment changes are specified for follow-up.