improvement(downloads): no-embed builtin presets + 1080p default

Embedding (thumbnail/chapters) rewrote the whole file — a 10 GB video spent minutes making a
10 GB temp copy — while the Plex .nfo + poster sidecars already carry that metadata. So:
- builtin presets no longer embed (migration 0040 re-seeds them; formats._DEFAULTS embed off);
  only the tiny FFmpegThumbnailsConvertor (for poster.jpg) remains, so downloads go
  video -> audio -> merge -> done with no full-file rewrite
- default preset is now 1080p (Best stays available, second in the list) so a long video
  doesn't silently pull ~10 GB and blow the quota
Users who want self-contained files can enable embedding in a custom profile.

backend/alembic/versions/0040_reset_builtin_profiles.py

@@ -0,0 +1,72 @@
+"""reset builtin download profiles: no embedding, 1080p default
+
+Revision ID: 0040_reset_profiles
+Revises: 0039_title_normalize
+Create Date: 2026-07-03
+
+Redefines the shipped presets: embedding (thumbnail/chapters/subs) is OFF (it rewrites the whole
+file — expensive on large videos — and the Plex .nfo/poster sidecars already carry the metadata),
+and the default is now 1080p (Best stays available but no longer first). Re-seeds the builtin
+rows so both fresh installs (0037 then this) and existing DBs converge on the same set. Existing
+jobs are unaffected (they snapshot their spec; profile_id just SET NULLs if it pointed at a
+replaced row).
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0040_reset_profiles"
+down_revision: Union[str, None] = "0039_title_normalize"
+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,
+    }
+
+
+# Default first (1080p), then Best, then the rest.
+_BUILTINS = [
+    ("1080p (MP4)", _spec("av", 1080, "mp4"), 10),
+    ("Best (MP4)", _spec("av", None, "mp4"), 20),
+    ("720p (MP4)", _spec("av", 720, "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 was seeded by 0037; not worth reconstructing.
+    pass