feat: open telemetry tracing for email send #663

Summary #

• Add tranquil-otel for optional OTLP export, TraceContext propagation, and trace_id/span_id on logs
• Wire tracing init into the server via [logging] config (RUST_LOG, JSON logs, OTLP endpoint/auth)
• Instrument admin send-email and HTTP requests with named spans

Why? #

When I was deploying my PDS I use grafana/loki for my observabiliy and had issues sending emails, tracing helped me sift through the logs to exactly the place I needed to find. Turns out I was doing something stupid but wanted to propose this as a helpful addition to the tranquil ecosystem.

Test plan #

• just fmt-check / just clippy / just lint-no-bsky
• just test (1464 passed)