feat: add ARMS tracing

[neta-zjj]

Summary

• Add OpenTelemetry tracing for Gin inbound requests and shared HTTP outbound requests.
• Propagate incoming traceparent through relay requests to upstream model services.
• Add trace/span fields to business and Gin logs, keeping existing request-id headers intact.

Tests

• go test ./pkg/armsotel ./middleware ./service ./relay/channel
• go test ./... (blocked by missing web/classic/dist and existing unrelated relay/channel/claude + relay/helper failures)