feat: .md download bundles thumbnails, auto-cache on channel add

- Bulk 'Download .md' now also caches thumbnails (one action does both);
  removed the redundant standalone Thumbnails button from the bulk action bar.
- Scrape UX optimization: batch job skips videos whose .md already exists
  (just caches their thumbnail) instead of re-extracting.
- Auto-thumbnails: on addChannel, thumbnails cache in the background so a
  newly-attached channel's catalog looks populated immediately, even before
  any .md is generated.
- Thumbnail fallback: pending videos without a stored thumbnail URL use the
  canonical https://i.ytimg.com/vi/<id>/hqdefault.jpg, so every video shows
  a thumbnail (and /api/thumbnails/{id} never 404s — redirects if uncached).
- New helpers in pipeline.py: thumbnail_url_for() + cache_thumbnail().

src/yt_scraper/webapp/static/index.html

@@ -213,13 +213,9 @@
           <template x-if="videos.selected.length > 0">
             <div class="bulk-bar glass p-3 flex flex-wrap items-center gap-2">
               <span class="pill accent-grad btn-primary font-bold" x-text="videos.selected.length + ' selected'"></span>
-              <button class="btn accent-grad btn-primary !py-1.5 !px-3 text-xs" @click="downloadMdBatch()">
+              <button class="btn accent-grad btn-primary !py-1.5 !px-3 text-xs" @click="downloadMdBatch()" title="Download .md + thumbnails for the selection">
                 <svg class="w-3.5 h-3.5" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path stroke-linecap="round" stroke-linejoin="round" d="M12 4v12m0 0l-4-4m4 4l4-4M4 18v2h16v-2"/></svg>
-                Download .md
-              </button>
-              <button class="btn btn-ghost !py-1.5 !px-3 text-xs" @click="downloadThumbnails()">
-                <svg class="w-3.5 h-3.5" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path stroke-linecap="round" stroke-linejoin="round" d="M3 16.5v2.25A2.25 2.25 0 005.25 21h13.5A2.25 2.25 0 0021 18.75V16.5M16.5 12L12 7.5 7.5 12M12 7.5V18"/></svg>
-                Thumbnails
+                Download .md + thumbs
               </button>
               <button class="btn btn-ghost !py-1.5 !px-3 text-xs" @click="downloadAudio()">
                 <svg class="w-3.5 h-3.5" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path stroke-linecap="round" stroke-linejoin="round" d="M9 18V6l10-2v12M9 18a3 3 0 11-6 0 3 3 0 016 0zm10-2a3 3 0 11-6 0 3 3 0 016 0z"/></svg>