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.

backend/app/downloads/formats.py

@@ -47,7 +47,9 @@ _DEFAULTS = {
 # Bump when the spec→output-bytes mapping changes so an identical spec gets a fresh cache
 # identity instead of hitting a stale asset produced by the old rule. v2: mp4 downloads now
 # prefer H.264+AAC (browser/iOS-compatible) rather than yt-dlp's default VP9/Opus best.
-_SIG_VERSION = 2
+# v3: mp4 compat now *selects* H.264 (avc1) with a progressive fallback + re-encodes the rare
+# AV1/VP9-only result to H.264, so cached AV1 assets must re-derive (fixes iOS broken-player).
+_SIG_VERSION = 3
 
 _SPONSORBLOCK_CATEGORIES = ["sponsor", "selfpromo", "interaction"]
 _VCODEC_SORT = {"h264": "vcodec:h264", "vp9": "vcodec:vp9", "av1": "vcodec:av01"}
@@ -142,22 +144,41 @@ def build_ydl_opts(
         )
     else:
         hcap = f"[height<=?{h}]" if h else ""
+        # Compatibility-first for mp4 (the universal container): produce H.264 video + AAC audio
+        # so shared/downloaded files play in EVERY browser, including iOS/Safari. WebKit (which
+        # every iOS browser, Brave included, is forced to use) decodes only H.264/H.265 + AAC
+        # inside mp4 — a VP9/AV1-in-mp4 file shows a broken player there, even though Chromium
+        # plays it fine. A custom profile that explicitly sets `vcodec` opts out.
+        compat = s["container"] == "mp4" and s["vcodec"] is None
         if s["mode"] == "v":
-            opts["format"] = f"bestvideo{hcap}/best{hcap}"
+            if compat:
+                opts["format"] = (
+                    f"bestvideo[vcodec^=avc1]{hcap}/best[vcodec^=avc1]{hcap}"
+                    f"/bestvideo{hcap}/best{hcap}"
+                )
+            else:
+                opts["format"] = f"bestvideo{hcap}/best{hcap}"
         else:  # av
-            opts["format"] = f"bestvideo{hcap}+bestaudio/best{hcap}"
+            if compat:
+                # Prefer H.264 at the SELECTION level, not just via format_sort: a sort key is only
+                # a *preference* and can't conjure an H.264 stream a source doesn't expose as
+                # video-only. A non-YouTube source (e.g. Facebook) may ship AV1 as the best
+                # video-only stream and H.264 only as a *progressive* (a+v) format — which
+                # `bestvideo` skips — so `bestvideo+bestaudio` picked AV1 and iOS broke. This chain
+                # tries avc1 video-only, then the avc1 progressive, then anything.
+                opts["format"] = (
+                    f"bestvideo[vcodec^=avc1]{hcap}+bestaudio[acodec^=mp4a]"
+                    f"/best[vcodec^=avc1]{hcap}"
+                    f"/bestvideo{hcap}+bestaudio/best{hcap}"
+                )
+            else:
+                opts["format"] = f"bestvideo{hcap}+bestaudio/best{hcap}"
         if s["container"]:
             opts["merge_output_format"] = s["container"]
         sort = []
-        # Compatibility-first for mp4 (the universal container): prefer H.264 video + AAC
-        # audio so shared/downloaded files play in EVERY browser, including iOS/Safari.
-        # WebKit (which every iOS browser, Brave included, is forced to use) decodes only
-        # H.264/H.265 + AAC inside mp4 — a VP9/Opus-in-mp4 file shows a broken player there,
-        # even though Chromium plays it fine. These sort keys rank codec ABOVE resolution, so
-        # a 720p H.264 stream is chosen over a 1080p-VP9-only one; VP9/AV1 remain a graceful
-        # fallback only when no H.264 stream exists. A custom profile that explicitly sets
-        # `vcodec` opts out (its codec choice wins over the compatibility default).
-        if s["container"] == "mp4" and s["vcodec"] is None:
+        # Keep the codec preference as a tiebreaker (ranks H.264/AAC above resolution among the
+        # selected candidates); the selector above is the hard guarantee, this is belt-and-braces.
+        if compat:
             sort += ["vcodec:h264", "acodec:aac"]
         if h:
             sort.append(f"res:{h}")