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/formats.py

@@ -57,6 +57,10 @@ _DEFAULTS = {
 # opt-out), so any asset cached under an old non-compat spec — or a stale non-H.264 mp4 that slipped
 # the v3 net — must re-derive through the current browser-safe guarantee.
 _SIG_VERSION = 4
+# Audio-only downloads version on their own, so a change to the audio bytes doesn't re-download every
+# cached video. v5: an extracted .mp3/.m4a is stored as itself instead of remuxed into an .mp4 that
+# iPad's WebKit refuses to play, so a cached remuxed audio asset must re-derive.
+_AUDIO_SIG_VERSION = 5
 
 _SPONSORBLOCK_CATEGORIES = ["sponsor", "selfpromo", "interaction"]
 
@@ -103,7 +107,7 @@ def format_sig(spec: dict, salt: str | None = None) -> str:
     always re-fetches into its own asset instead of sharing a cached file with other jobs)."""
     s = normalize(spec)
     data = {k: s[k] for k in _SIG_KEYS}
-    data["_v"] = _SIG_VERSION
+    data["_v"] = _AUDIO_SIG_VERSION if s["mode"] == "a" else _SIG_VERSION
     if salt is not None:
         data["_salt"] = salt
     payload = json.dumps(data, sort_keys=True, separators=(",", ":"))