Keep two changes to the catalogue made at once from undoing each other (#153)

Each change to the catalogue or the server settings stored the whole thing from a copy it took
earlier, and nothing kept two from overlapping. Adding a feed copied it first and then spent up to
a minute finding and reading the feed, so another person's new feed, an unsubscribe, an admin's
settings, or the scan's own changes (a moved feed's address, a category) made in that minute were
stored over when it finished, with no error. A feed dropped that way left its subscriber's list.

Every change now takes its copy through Ctx::edit_cfg, which holds a lock until store_cfg keeps it,
and store_cfg takes nothing else, so no writer can skip it. Adding a feed takes its copy once it
has found the feed, and before the duplicate check, so two people adding one feed add it once.
reload takes the lock too. `ipx add`, `rm` and `import`, in a process of their own, are outside it,
as the comment on the lock says.

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

CHANGELOG.md

@@ -15,6 +15,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 
+- Changes to the feed list made at the same moment, such as two people adding feeds, or a feed added while the server settings change, no longer undo one another.
 - A download that stops partway is given up after two minutes without data, instead of holding up every scan and download until a restart.
 - On a phone, the closed feed list no longer shades the left edge of the page with its shadow.
 - On an iPhone, the top bar's colour runs all the way up behind the status bar and the Dynamic Island, with no band of another colour above it.