Trace each step of the feed list (#164)

GET /api/feeds took 250-300 ms in production with nothing under it in its trace but feed_summary
spans of a millisecond, so there was no telling where the time went. feed_rows now wraps the
catalogue, the person's subscriptions, the subscriber counts, the pins and feed_list in a step
span each, and feed_list each of its five queries.

db::step makes a span only inside one already: the healthcheck's status every 30 s and the page's
live updates run the same code outside any trace, where each span would be a trace of its own, as
feed_summary's already are.

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
 
 ### Changed
 
+- The feed list's traces show each of its database queries, so it can be told where its time goes.
 - Saving a feed's settings asks the database for your subscription once, not twice.
 - `ipx add` and Add feed in the page add a feed the same way, through one piece of code.
 - Glass's top bar is solid on a phone, in the colour its glass showed, so the strip above it on an iPhone can match it.