fix(downloads): a signed-in Facebook reel keeps its title, and an mp3 stays an mp3

Since a Facebook cookie was added on prod, every reel came down titled
"Video": measured on dev, a signed-in read gives yt-dlp the placeholder
"Video" (an 18+ reel read as Chrome gives "Facebook") while the post's text
is still in the description. A new SourceAdapter.title hook takes the whole
first line of the description in that case, and both title writes (the early
fill and the finished download) go through it, so the row, the file name and
the share preview carry the post's text again. The line is not cut with an
ellipsis: normalize_title's NFKC turns "…" into "..." and collapses it to
".", which read as a finished sentence (seen in dev UAT).

The "Audio (MP3)" profile's extracted .mp3 was remuxed into an .mp4 by the
browser-safe step (the audio spec is clamped to container mp4 too) and served
as video/mp4, which iPad's WebKit will not play (prod job 268). Audio mode now
skips that step, so an .mp3 is served as audio/mpeg and an .m4a as audio/mp4.
Audio downloads version their cache identity on their own (v5), so a cached
remuxed audio re-derives without re-downloading every cached video.

backend/app/downloads/pipeline.py

@@ -113,9 +113,10 @@ def _fill_asset_meta_early(asset_id: int, info: dict) -> None:
         # `or info.get("id")` avoids a blank card while downloading when the cleaned title is empty
         # (e.g. a title that was only a social engagement prefix); the completion path later sets
         # the final title with its own source_ref fallback.
-        asset.title = normalize_title(info.get("title")) or info.get("id")
+        adapter = adapters.resolve_adapter(None, None, info)
+        asset.title = normalize_title(adapter.title(info)) or info.get("id")
         asset.uploader = info.get("uploader") or info.get("channel")
-        asset.uploader_url = adapters.resolve_adapter(None, None, info).uploader_url(info)
+        asset.uploader_url = adapter.uploader_url(info)
         asset.thumbnail_url = info.get("thumbnail")
         asset.duration_s = int(info["duration"]) if info.get("duration") else None
         asset.upload_date = _parse_upload_date(info.get("upload_date"))
@@ -403,8 +404,12 @@ def _ensure_browser_playable(job_id: int, produced: Path, spec: dict, info: dict
     or just remux when the codecs are fine but the container isn't. **Scope:** only the mp4 compat
     profile is normalized; an explicit `vcodec` or a non-mp4 custom profile is the user opting out
     of the guarantee and is left as-is (so a `webm` profile can still ship VP9/Opus). Returns the
-    file to store (converted, or the original) and corrects `info`'s codec fields to match disk."""
-    if spec.get("container") != "mp4" or spec.get("vcodec"):
+    file to store (converted, or the original) and corrects `info`'s codec fields to match disk.
+
+    An audio download (mode "a") is never touched: yt-dlp already extracted it to .mp3 / .m4a, which
+    every browser plays as audio/mpeg / audio/mp4. Remuxing it into .mp4 served an mp3 as video/mp4,
+    which iPad's WebKit refuses to play (prod job 268)."""
+    if spec.get("mode") == "a" or spec.get("container") != "mp4" or spec.get("vcodec"):
         return produced
     vcodec, acodec = editmod.probe_codecs(produced)
     is_mp4 = produced.suffix.lower() == ".mp4"
@@ -509,7 +514,7 @@ def _download(
         vcodec, acodec, width, height, duration_s = probe.resolve_media_facts(produced, info)
         meta = storage.MediaMeta(
             video_id=info.get("id") or source_ref,
-            title=normalize_title(info.get("title")) or source_ref,
+            title=normalize_title(adapter.title(info)) or source_ref,
             uploader=info.get("uploader") or info.get("channel") or "Unknown Channel",
             upload_date=_parse_upload_date(info.get("upload_date")),
             uniqueid_type=adapter.nfo_uniqueid_type(),