feat(mime): open audio files in Amberol, not mpv

mpv (video) and amberol (audio) are both already in the template suite;
the gap was mime wiring — audio/* still resolved to mpv.desktop. Repoint
the audio types to io.bassi.Amberol.desktop (id verified against the
built package) and broaden coverage to mp3/flac/ogg/opus/wav/m4a/aac,
both canonical and x- names. Video stays on mpv. Degrades to mpv if a
downstream drops Amberol from the suite.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

agent/BACKLOG.md

@@ -256,13 +256,6 @@ window. Wire `on-click-right` in `waybar.nix` AND the whole-swap jsoncs
 (parity rule); add a Hyprland `windowrulev2 = float` for the mixer.
 Ship the mixer in the template's `home.packages`. Pairs with item 41.
 
-### 37. Default video + audio players + mime associations
-(Raised by Bernardo, 2026-07-07 — item 7.) Install a video player and an
-audio player by default in `templates/downstream/home.nix`, and wire the
-mime defaults (item 8's `xdg.mimeApps` surface) so audio/video files open
-in them. (mpv already the video default per HARDWARE-QUEUE — confirm and
-extend to audio; pick the audio player.)
-
 ### 41. Floating-window audit
 (Raised by Bernardo, 2026-07-07 — item 11.) Decide which windows should
 open floating by default (config dialogs, pavucontrol, file-chooser