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

@@ -92,7 +92,8 @@ class BTreeDB(IPlaylistDB):
         return self.db[pos]
 
     def _setPlaylistPos(self, tag, pos, flush=True):
-        self.db[self._keyPlaylistPos(tag)] = pos.removeprefix(self._keyPlaylistStart(tag))
+        assert pos.startswith(self._keyPlaylistStart(tag))
+        self.db[self._keyPlaylistPos(tag)] = pos[len(self._keyPlaylistStart(tag)):]
         if flush:
             self._flush()
 
@@ -123,7 +124,9 @@ class BTreeDB(IPlaylistDB):
 
     def _getNextTrack(self, tag, pos):
         _, end_key = self._keyPlaylistStartEnd(tag)
-        return next(self.db.keys(pos, end_key))
+        it = self.db.keys(pos, end_key)
+        next(it)
+        return next(it)
 
     def getPlaylistForTag(self, tag: bytes):
         """

software/tests/utils_test/test_btreedb.py

@@ -15,7 +15,7 @@ class FakeDB:
     def values(self, start_key=None, end_key=None, flags=None):
         res = []
         for key in sorted(self.contents):
-            if start_key is not None and start_key >= key:
+            if start_key is not None and start_key > key:
                 continue
             if end_key is not None and end_key < key:
                 break
@@ -24,7 +24,7 @@ class FakeDB:
 
     def keys(self, start_key=None, end_key=None, flags=None):
         for key in sorted(self.contents):
-            if start_key is not None and start_key >= key:
+            if start_key is not None and start_key > key:
                 continue
             if end_key is not None and end_key < key:
                 break