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/config.yml.example

@@ -178,8 +178,10 @@ figures:
 # or any digit within it, for "unassigned" - a fresh install boots with none of this
 # and that is not an error. Editable from the web front-end, which writes back here.
 #
-#   target_kind: album   -> always starts from the first track (music, audiobooks).
-#                            target is an album id, as shown at GET /api/library.
+#   target_kind: album   -> starts from the first track (music, audiobooks), or from
+#                            track_index (0-based) when given - one song, followed by
+#                            the rest of the album. target is an album id, as shown
+#                            at GET /api/library.
 #   target_kind: series  -> always plays the newest episode of a podcast show, resolved
 #                            fresh on every press - never a fixed episode. target is the
 #                            show's folder name under Kinderpodcasts, e.g. "Wissen macht Ah".
@@ -190,3 +192,7 @@ remote:
   "2":
     target_kind: series
     target: "Wissen macht Ah"
+  "3":
+    target_kind: album
+    target: "3f9a0c12ab44"
+    track_index: 4