playlistdb: micropython does not have bytes.removeprefix()

- Increase the formatting of playlist entries to allow up to 100000
  tracks. Also enforce that playlist entries are indexed by integers
  using the validate method.

- Add a validate method to validate the data stored in the
  btreedb. Optionally dump the contents to stdout. For testing, add a
  validate+dump by default when opening the db. This can be removed once
  the playlistdb is validated.

software/src/utils/playlistdb.py

@@ -65,7 +65,7 @@ class BTreeDB(IPlaylistDB):
 
     @staticmethod
     def _keyPlaylistEntry(tag, pos):
-        return b''.join([tag, b'/playlist/', "{:05}".format(pos).encode()])
+        return b''.join([tag, b'/playlist/', '{:05}'.format(pos).encode()])
 
     @staticmethod
     def _keyPlaylistStart(tag):
@@ -159,6 +159,7 @@ class BTreeDB(IPlaylistDB):
         result = True
         last_tag = None
         last_pos = None
+        index_width = None
         for k in self.db.keys():
             fields = k.split(b'/')
             if len(fields) <= 1:
@@ -180,11 +181,15 @@ class BTreeDB(IPlaylistDB):
                     print(f'Malformed playlist entry: {k!r}')
                     result = False
                     continue
+                if index_width is not None and len(fields[2]) != index_width:
+                    print(f'Inconsistent index width for {last_tag} at {idx}')
+                    result = False
                 if (last_pos is not None and last_pos + 1 != idx) or \
                    (last_pos is None and idx != 0):
                     print(f'Bad playlist entry sequence for {last_tag} at {idx}')
                     result = False
                 last_pos = idx
+                index_width = len(fields[2])
                 if dump:
                     print(f'\tTrack {idx}: {self.db[k]!r}')
             elif fields[1] == b'playlistpos':