fix(downloads): make shared files iOS-playable + default to 720p

Shared /watch links showed a "broken play button" on iPad (any iOS
browser — all forced onto WebKit) while playing fine on desktop Chrome.
Root cause: the stored mp4's video codec was AV1 (confirmed via ffprobe),
which WebKit can't decode. The existing H.264 guard was only a
format_sort *preference*, so a non-YouTube source (Facebook) that ships
AV1 as its best video-only stream slipped through.

- B1: for the mp4 compat profile, prefer H.264 at the SELECTION level
  with a progressive fallback (bestvideo[vcodec^=avc1]+bestaudio /
  best[vcodec^=avc1] / anything) so an avc1 progressive is chosen over
  an AV1-only video-only stream.
- B2: re-encode the rare AV1/VP9-only result to H.264+AAC (+faststart)
  in the worker before storing, gated on the compat profile; corrects
  the asset codec metadata to match.
- Bump _SIG_VERSION 2->3 so cached AV1 assets re-derive.
- Default download preset is now 720p (migration 0058 re-seeds builtins
  720p-first; ProfileEditor BLANK 1080->720).
- Tests for the selector, compat opt-outs, and the transcode predicate.

frontend/src/components/ProfileEditor.tsx

@@ -11,7 +11,7 @@ const HEIGHTS = [null, 2160, 1440, 1080, 720, 480, 360];
 
 const BLANK: DownloadSpec = {
   mode: "av",
-  max_height: 1080,
+  max_height: 720,
   container: "mp4",
   audio_format: "m4a",
   vcodec: null,