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/alembic/versions/0058_dl_default_720p.py

@@ -0,0 +1,71 @@
+"""reset builtin download profiles: 720p is now the default (first) preset
+
+Revision ID: 0058_dl_default_720p
+Revises: 0057_plex_ppl_enriched
+Create Date: 2026-07-22
+
+The download dialog preselects the first builtin (lowest sort_order). Make 720p that default —
+it plays everywhere, downloads faster, and is plenty for the shared /watch player — with 1080p and
+Best still one click away. Re-seeds the builtin rows so fresh installs and existing DBs converge
+(same mechanism as 0040). Existing jobs are unaffected (they snapshot their spec; a profile_id that
+pointed at a replaced row just SET NULLs).
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0058_dl_default_720p"
+down_revision: Union[str, None] = "0057_plex_ppl_enriched"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def _spec(mode, height=None, container=None, audio_format=None):
+    return {
+        "mode": mode,
+        "max_height": height,
+        "container": container,
+        "audio_format": audio_format,
+        "vcodec": None,
+        "embed_subs": False,
+        "embed_chapters": False,
+        "embed_thumbnail": False,
+        "sponsorblock": False,
+    }
+
+
+# 720p first (the default preselection), then 1080p, Best, then the rest.
+_BUILTINS = [
+    ("720p (MP4)", _spec("av", 720, "mp4"), 10),
+    ("1080p (MP4)", _spec("av", 1080, "mp4"), 20),
+    ("Best (MP4)", _spec("av", None, "mp4"), 30),
+    ("Audio (M4A)", _spec("a", None, None, "m4a"), 40),
+    ("Audio (MP3)", _spec("a", None, None, "mp3"), 50),
+    ("Video only (MP4)", _spec("v", None, "mp4"), 60),
+]
+
+_profiles = sa.table(
+    "download_profiles",
+    sa.column("user_id", sa.Integer),
+    sa.column("name", sa.String),
+    sa.column("spec", sa.JSON),
+    sa.column("is_builtin", sa.Boolean),
+    sa.column("sort_order", sa.Integer),
+)
+
+
+def upgrade() -> None:
+    op.execute("DELETE FROM download_profiles WHERE is_builtin = true")
+    op.bulk_insert(
+        _profiles,
+        [
+            {"user_id": None, "name": n, "spec": s, "is_builtin": True, "sort_order": o}
+            for n, s, o in _BUILTINS
+        ],
+    )
+
+
+def downgrade() -> None:
+    # No-op: the previous builtin set (1080p first) was seeded by 0040; not worth reconstructing.
+    pass