fix(downloads): show real title + thumbnail while queued/downloading

A queued/downloading row previously showed the bare video id and no thumbnail because asset
metadata was only filled when the download completed. Now:
- service.populate_from_catalog fills title/uploader/thumbnail/date/duration at enqueue from
  our own Video/Channel catalog (0 network cost) — feed downloads look right instantly
- worker fills the same from yt-dlp's info_dict on the first progress event (covers ad-hoc
  URLs / catalog misses), best-effort, never fails a download

Verified: a paused feed download now shows its real title, channel, thumbnail and duration.

backend/app/downloads/service.py

@@ -13,7 +13,7 @@ from sqlalchemy.exc import IntegrityError
 from sqlalchemy.orm import Session
 
 from app.downloads import formats, quota
-from app.models import DownloadJob, MediaAsset
+from app.models import Channel, DownloadJob, MediaAsset, Video
 
 
 def source_url(source_kind: str, source_ref: str) -> str:
@@ -54,6 +54,27 @@ def get_or_create_asset(
     return asset
 
 
+def populate_from_catalog(db: Session, asset: MediaAsset) -> bool:
+    """Fill an asset's display metadata (title/uploader/thumbnail/date/duration) from our own
+    catalog when the source is a known YouTube video — so a queued/downloading row shows the
+    real title + thumbnail immediately instead of the bare id. 0 network cost. Returns True if
+    it found a catalog match."""
+    if asset.source_kind != "youtube" or asset.title:
+        return False
+    video = db.get(Video, asset.source_ref)
+    if video is None:
+        return False
+    asset.title = video.title
+    asset.thumbnail_url = video.thumbnail_url
+    asset.duration_s = video.duration_seconds
+    if video.published_at:
+        asset.upload_date = video.published_at.date()
+    channel = db.get(Channel, video.channel_id)
+    if channel is not None:
+        asset.uploader = channel.title
+    return True
+
+
 def _next_queue_pos(db: Session) -> int:
     return (db.execute(select(func.coalesce(func.max(DownloadJob.queue_pos), 0))).scalar() or 0) + 1
 
@@ -74,6 +95,8 @@ def enqueue(
     spec = formats.normalize(spec)
     sig = formats.format_sig(spec)
     asset = get_or_create_asset(db, source_kind, source_ref, sig)
+    # Show a real title + thumbnail the moment it's queued (feed downloads are catalog videos).
+    populate_from_catalog(db, asset)
 
     job = DownloadJob(
         user_id=user_id,