Send OpenTelemetry traces over OTLP (#87)

ipx had no spans, only log lines, so there was no way to see where a
slow scan, download or request spent its time. With
OTEL_EXPORTER_OTLP_ENDPOINT set, the daemon now exports traces over
OTLP/HTTP (Tempo on Tower): a scan, each feed in it, the feed fetch and
site icon lookup, downloads, torrents, reaps, and web requests. Log lines
inside a span ride along as its events.

Only the daemon exports: the healthcheck runs ipx status every 30s and
would bury everything else. The web event stream and the log view's
polling get no span, for the same reason. The exporter shares ipx's
reqwest 0.13, so no second HTTP stack comes in.

The stderr log now prefixes lines inside a span with it, as
tracing-subscriber's fmt layer does (scan{only=None force=false}: ...).

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

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- With `OTEL_EXPORTER_OTLP_ENDPOINT` set, the daemon sends traces of its scans, downloads and web
+  requests to a collector such as Tempo.
 - A feed that has no artwork of its own shows its website's icon instead.
 - The refresh button turns while its feed is being checked, and the check-every-feed buttons
   while any of yours is.