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>

docs/configuration.md

@@ -143,4 +143,5 @@ and they are re-derived on every scan. Editing one in the UI promotes it to a ca
 | `IPX_TEST_DATABASE_URL` | For `cargo test`: run the database tests on this Postgres database too, each in a schema of its own |
 | `IPX_LOG` | What reaches stderr (`ipx=debug`, `ipx::scan=debug`, …) |
 | `IPX_UI_LOG` | What the in-process log buffer captures for the UI's Log view |
+| `OTEL_EXPORTER_OTLP_ENDPOINT` | An OTLP/HTTP collector, such as Tempo at `http://host:4318`: the daemon sends it traces of scans, downloads and web requests. The other `OTEL_EXPORTER_OTLP_*` variables apply too |
 | `http_proxy` / `https_proxy` | Honoured for feed and enclosure fetches |