Add distributed tracing (OpenTelemetry)

## Description

The middleware has no distributed tracing. For a multi-service system (Relayer ↔ Canton ↔ Ethereum ↔ API Server ↔ Indexer), traces would significantly improve debugging of cross-service latency and failures.

Scope:

- Add go.opentelemetry.io/otel dependency
- Instrument outgoing HTTP calls (Canton Ledger API, Ethereum RPC) with trace propagation
- Instrument incoming HTTP handlers with OTEL middleware
- Add a Jaeger or OTEL Collector service to docker-compose (under the monitoring profile)
- Export traces via OTLP

Acceptance Criteria:

- [ ] Trace spans are created for bridge transfer lifecycle (receive event → submit tx → confirm)
- [ ] Trace context propagated across HTTP calls
- [ ] Jaeger UI accessible locally to view traces
- [ ] Config toggle to enable/disable tracing (tracing.enabled)