refactor(playlistdb): Enforce constant entry index length

To simplify the playlist handling, enforce that the indices are always
formatted to the same length (5, which allows for 100000 entries, that
should be enough).

Then make the position stored in the Playlist object be a simple integer
instead of a database key. This simplifies the code, and will make
implementing shuffle much easier.

Signed-off-by: Matthias Blankertz <matthias@blankertz.org>

DEVELOP.md

@@ -18,10 +18,8 @@ pip install -U micropython-rp2-pico_w-stubs --target typings
 The playlist data is stored in the btree database in a hierarchical schema. The hierarchy levels are
 separated by the '/' character.  Currently, the schema is as follows: The top level for a playlist
 is the 'tag' id encoded as a hexadecimal string. Beneath this, the 'playlist' key contains the
-elements in the playlist. The keys used for the playlist entries should be decimal integers,
-prefixed with sufficient zeros such that they are in the correct order when sorted
-lexicographically. All keys must have the same width. When writing a playlist using the playlistdb
-module, the keys are 00000, 00001, etc. by default.
+elements in the playlist. The keys used for the playlist entries must be decimal integers,
+left-padded with zeros so their length is 5 (e.g. format `{:05}`).
 
 #### Playlist modes