Number keys: the keyboard's digits work like the remote's; a key can hold one song

The IR remote's digits and the kiosk keyboard's number keys now both emit
PlayRemoteSlotRequested, and one reaction resolves the slot for either (a podcast
show to its newest episode, an album to the album). POST /api/remote/press/{digit}
is the keyboard's way in. Mid-search a digit is still part of the query.

A remote slot can start one song instead of a whole album: `track_index` on an
album slot plays from that track, then the rest of the album. Shift+A on the play
screen arms "this song" where A arms "this album", and the assign popup offers
both. A series slot cannot name a song, and an index past the album's end is
rejected when saving.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

python-backend/README.md

@@ -217,6 +217,8 @@ copied to the device.
 | `POST /api/play` | `{album_id, track_index?}` |
 | `POST /api/resume` `/pause` `/next` `/previous` `/seek` | transport |
 | `POST /api/volume` | `{percent}` or `{delta_percent}` |
+| `POST /api/remote/press/{digit}` | what the IR remote's number key does - the keyboard's digits |
+| `GET` `PUT /api/remote/mapping` | number key -> album (optionally from one `track_index`) or podcast show |
 | `GET` `PUT /api/settings` | parent mode |
 
 Two decisions worth knowing: