Add OpenTelemetry trace export support

[irparent]

Iris currently stores traces in SQLite. Many teams already have observability stacks (Datadog, Grafana, New Relic) that ingest OpenTelemetry spans.

Goal: Add an optional OTel exporter that converts Iris traces to OTel spans and sends them to any OTLP-compatible endpoint.

Acceptance criteria:

• New CLI flag --otel-endpoint to configure the OTLP endpoint
• Traces logged via log_trace are also exported as OTel spans
• Spans include GenAI semantic conventions (model, token counts, cost)
• Existing SQLite storage continues to work alongside export

Resources:

• OTel GenAI semantic conventions
• Architecture guide
• API reference