Traces first (CLAUDE.md); ipx-web traces its requests (#213)

Caddy's tracing directive, switched by WEB_TRACING so a server without a collector does not log
a failed export every few seconds. It continues an incoming traceparent (the page starts one per
tap, #190) and passes it to ipx, so a request is one trace from the browser through Caddy into
ipx, and Caddy's access log carries its traceID. Checked against Tempo with a probe container:
service.name and deployment.environment.name come from the OTEL_ variables, over OTLP/HTTP.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- ipx-web sends a trace span for each request when `WEB_TRACING=on`, continuing the page's trace into ipx, so one trace runs from the browser through Caddy to ipx (#213).
 - The Grafana dashboard is laid out by container: an overview of all three, Tower's load, CPU, memory and disks, then ipx-web (from its new request log), ipx's API, worker and process, and the directory, each with its own warnings (#210).
 - `ipx-directory`, a directory of public podcasts that ipx servers share: each sends the public feeds its people subscribe to, and any server with a token browses them. Tokens are in its config file, each read-only, read-write or admin, and revoked one at a time (#204).