Send a changed feed's row to the page instead of it reloading the list (#105)

After a feed was checked, failed or downloaded a file, and after every item read, the page
fetched /api/feeds whole, about 60 ms for 160 rows, though one row had changed. The live event
stream now knows who is connected and, after an event that changes a feed, sends that person
its row (feed_row), built by the same code as the list (feed_rows, with Db::feed_list asked for
one feed). Marking an item read answers with the feed's row. The page puts the row in place
and redraws once a frame. A routine skip of a feed not due sends nothing.

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

CHANGELOG.md

@@ -23,6 +23,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The feed list updates just the feed that changed, as a scan checks it or you read an item, instead of reloading the whole list.
 - A scan fetches several feeds at once, so a refresh no longer waits on every site in turn.
 - An open page reloads the feed list only when a scan has checked something, not every minute.
 - Adding an address checks it first: a feed is added, a web page adds the feed it links, and anything else is refused with the reason, instead of being added and failing on every check.