Pin a feed to the top of the feed list

subscriptions.pinned, per person, set by PATCH /api/feeds/{id} {pinned} and
returned as FeedRow.pinned. Kept out of Sub, which the scanner merges into its
policy; set_subscription names its columns, so saving a feed's settings leaves
the pin alone (tested).

Pinned feeds come first in the list, a pin before the name and a rule under the
block: a pinned folder with its feeds under it, a feed from inside one lifted out
of it. The pin button is on both the feed and the folder page.

Closes #33.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CHANGELOG.md

@@ -19,6 +19,8 @@ The long form, with what was wrong before and how it was found, is in
 - Your theme is kept on your account rather than in the browser, so it follows you to another
   browser or computer, and the page arrives in it with no flash of the default. The theme a
   browser already had is saved to your account the first time you load the page.
+- Pin a feed to the top of the feed list with the pin on its page, a feed from inside an OPML or
+  Patreon folder included, which comes out of the folder while pinned. Pins are yours alone.
 - Touch gestures: pull the item list down from its top to check the feed for new items, and
   swipe the item you are reading left for the next one and right for the one before, or back
   to the list from the first.