Announce a followed move as a feed_moved event (#115)

A feed moved to its new address was only logged by follow_move. It is now an event, feed_moved
with the feed and its old and new addresses, so it goes where every other event goes: the log,
with from and to as fields, the admin page's Scans view, `ipx fetch`, and the page, which gets
the feed's new row.

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

CHANGELOG.md

@@ -18,7 +18,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
-- A feed that has moved for good (a permanent redirect) is followed to its new address, which iPX then reads from. A temporary redirect changes nothing.
+- A feed that has moved for good (a permanent redirect) is followed to its new address, which iPX then reads from, and says so in the log as `feed_moved`. A temporary redirect changes nothing.
 - The daemon sleeps until the next feed is due, at most ten minutes, instead of looking every minute; refreshing or adding a feed still wakes it at once.
 - A pinned feed's pin sits on the corner of its artwork, as a failing feed's mark does, instead of before its name.
 - The Directory lists feeds nobody subscribes to yet; Popular still lists what people subscribe to.