Add a feed through one function, from the CLI and the page alike (#161)

`ipx add` and the page's Add feed were the same steps written twice: find the feed, look for it in
the catalogue by same_feed, add_one, store. They had drifted, and #153's lock had to be put into
both by hand. add_to_catalogue() now does the shared part, under the lock, and answers the id and
whether the feed was there already; each caller still finds the feed itself, so the page can call a
bad address the caller's mistake (400), and then does its own part: the CLI lists or categorises,
the page subscribes and scans. add_one is private to it now.

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
 
+- `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.
 - A pinned item or feed's pin button is filled with the theme's accent, in the reader, on the feed's page and in the toolbar, so pinned no longer looks like not pinned.
 - A feed's page in the Directory says how many people here subscribe only when someone does, without "Nobody here subscribes yet".