Race the CDN and the proxy, for thumbnails and for playback

A thumbnail used to try the provider and only ask /api/image once that
had failed, so every hotlink-blocked host cost a wasted request per card
before anything appeared. Both routes now go out together for the first
thumbnail of a host, and the rest of the batch waits on that one answer
rather than each rediscovering it. Speed decides which image is shown;
capability decides what the host is remembered as, since the proxy tends
to win first contact merely for being same-origin -- pinning a host to it
over that would push a whole page of thumbnails through our own server.

Playback asks the same question, but per video and at play time: one
provider can spread its media over several CDNs, so there is nothing
useful to pre-compute, and the old per-card probe answered for whichever
card happened to scroll past. The direct route is now tested alongside
the proxied playback and takes over if it answers before a frame is
decoded. Whatever loses is cancelled -- the token guards stopped stale
callbacks but left their requests running, so the losing route kept
pulling bytes and the server kept an upstream connection open for them.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01QBDkEXP4htyXTCZUwMLphd

frontend/js/enhance.js

@@ -79,7 +79,7 @@ App.enhance = App.enhance || {};
             const ready = meta && Array.isArray(meta.formats) && meta.formats.length;
             if (!ready) {
                 // Not resolved yet: kick it off so the *next* hover can preview.
-                if (typeof App.videos.resolveAndProbe === 'function') App.videos.resolveAndProbe(v);
+                if (typeof App.videos.ensureFormats === 'function') App.videos.ensureFormats(v);
                 return;
             }
             let url = '';