feat(relay): implement OpenTelemetry baseline tracing (MM-137)
- Add `[telemetry]` config section (enabled, otlp_endpoint, service_name)
with env overrides: EZPDS_TELEMETRY_ENABLED, EZPDS_OTLP_ENDPOINT,
OTEL_SERVICE_NAME
- New crates/relay/src/telemetry.rs: layered subscriber init with
conditional OTel layer; OtelGuard flushes spans on graceful shutdown
- OtelMakeSpan in app.rs: custom TraceLayer make_span_with that extracts
W3C traceparent/tracestate headers and links upstream traces via
TraceContextPropagator
- DB instrumentation: #[tracing::instrument] on open_pool and
run_migrations with db.system = "sqlite" span attribute
- 7 new config tests covering telemetry TOML parsing, all three env
overrides, env-wins-over-toml precedence, and invalid bool error
- Zero overhead when telemetry.enabled = false (default)