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/api.py

@@ -332,41 +332,29 @@ def build_router(store: Store, cfg: Config, jobs) -> APIRouter:
     # -------------------------------------------------- thumbnails (local cache)
     @r.post("/tools/thumbnails")
     async def download_thumbnails(payload: dict):
-        import requests as _req
+        from ..pipeline import cache_thumbnail
         video_ids = (payload or {}).get("video_ids") or []
         channel_id = (payload or {}).get("channel_id")
         if channel_id and not video_ids:
-            video_ids = [v.video_id for v in store.get_all(channel_id) if v.thumbnail]
+            video_ids = [v.video_id for v in store.get_all(channel_id)]
         out_dir = Path(cfg.output_dir_resolved).parent / "thumbnails"
         out_dir.mkdir(parents=True, exist_ok=True)
         n = 0
         for vid in video_ids:
-            v = store.get_video(vid)
-            if not v or not v.thumbnail:
-                continue
-            target = out_dir / f"{vid}.jpg"
-            if target.exists():
+            if cache_thumbnail(store, vid, out_dir):
                 n += 1
-                continue
-            try:
-                r2 = _req.get(v.thumbnail, timeout=15, headers={"User-Agent": "Mozilla/5.0"})
-                r2.raise_for_status()
-                target.write_bytes(r2.content)
-                n += 1
-            except Exception:
-                continue
         return {"downloaded": n, "dir": str(out_dir)}
 
     @r.get("/thumbnails/{video_id}")
     def serve_thumbnail(video_id: str):
         from fastapi.responses import RedirectResponse
+        from ..pipeline import thumbnail_url_for
         local = Path(cfg.output_dir_resolved).parent / "thumbnails" / f"{video_id}.jpg"
         if local.exists():
             return FileResponse(str(local), media_type="image/jpeg")
         v = store.get_video(video_id)
-        if v and v.thumbnail:
-            return RedirectResponse(url=v.thumbnail, status_code=302)
-        raise HTTPException(404, "no thumbnail")
+        url = thumbnail_url_for(v) if v else f"https://i.ytimg.com/vi/{video_id}/hqdefault.jpg"
+        return RedirectResponse(url=url, status_code=302)
 
     # -------------------------------------------------- clip (transcript segment)
     @r.get("/clip/{video_id}")