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>

web/src/feeds.ts

@@ -1,4 +1,13 @@
 /* ---------------- feeds ---------------- */
+/// One feed's row from the server, in place of the old one (or added, for a feed an OPML just
+/// listed). A scan sends dozens; the list is redrawn once a frame, not once each.
+let rowsQueued=0;
+function patchFeed(row){
+  const i=S.feeds.findIndex(f=>f.id===row.id);
+  if(i<0) S.feeds.push(row); else S.feeds[i]=row;
+  if(!rowsQueued) rowsQueued=requestAnimationFrame(()=>{ rowsQueued=0; renderFeeds(); });
+}
+
 async function loadFeeds(keepSel?: boolean){
   S.feeds = await api('/api/feeds');
   // Only for a hint in a feed's settings, so once, on the page's first load, not on every reload