Rename everything to Stemify, and add a README

- Python package stemroller/ -> stemify/, Android package com.stemroller.mobile ->
  com.stemify.mobile, app names, QML module, icons, desktop entries, MPRIS and Plex
  product names
- Done-markers are now .stemify.json; stemd renames the old ones once at startup so
  songs separated before the rename aren't separated again
- A top-level README covering the server, the Android and Android Auto app, the EQ,
  Stem Now and the desktop apps

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -2,5 +2,5 @@
 *
 !server/stemd.py
 !server/stemd-requirements.txt
-!stemroller/*.py
-!stemroller/worker/*.py
+!stemify/*.py
+!stemify/worker/*.py