fix: remove playlists left behind by the switch to .m3u8

Every playlist appeared twice on the device. Pruning is driven by a record of
what was written last time, deliberately, so that a playlist put in that
directory by hand is never touched. But the record cannot reach back before it
existed: the .m3u files were written by a version that kept none, so when the
extension changed there was nothing to prune them by, and rsync carried both
copies across.

A file with the same name as one being written now, under an extension this
tool used to write, is removed as well. That is narrow enough to match only its
own leavings -- a hand-made playlist has a name this tool never produces, so it
survives whatever its extension, and there is a test for that alongside the one
for the duplicate.

music_curator.py

@@ -75,6 +75,11 @@ PLAYLIST_DIRECTORY = "_playlists"
 # library holds Mötley Crüe, Beyoncé and Sigur Rós.
 PLAYLIST_SUFFIX = ".m3u8"
 
+# Extensions this tool has written in the past. A playlist of the same name
+# under one of these is a leftover of its own, and is cleaned up even though no
+# record of writing it survives.
+SUPERSEDED_SUFFIXES = (".m3u",)
+
 # Tags are re-fetched this often. They move slowly, and the first pass over a
 # library already costs one request per artist.
 TAG_REFRESH_SECONDS = 90 * 86400
@@ -1674,9 +1679,22 @@ def prune_playlists(directory, produced, store):
     Driven by a record of what was written last time rather than by "every M3U
     that is not one of ours", so a playlist put there by hand is left alone. A
     renamed mood otherwise leaves its old file on the device for ever.
+
+    That record cannot reach back before it existed, which is how the switch
+    from .m3u to .m3u8 left every playlist on the device twice: the old files
+    were written by a version that kept no record, so there was nothing to
+    prune them by. A file with the same name as one being written now, under a
+    superseded extension, is therefore removed as well -- narrow enough that
+    only this tool's own leavings match it.
     """
     previous = set(json.loads(store.get_state("playlist_files") or "[]"))
-    for name in sorted(previous - set(produced)):
+    stale_names = previous - set(produced)
+    for name in produced:
+        for superseded in SUPERSEDED_SUFFIXES:
+            stale_names.add(Path(name).with_suffix(superseded).name)
+    stale_names -= set(produced)
+
+    for name in sorted(stale_names):
         stale = directory / name
         if stale.is_file():
             logger.info("removing playlist %s, no longer produced", name)