Reorganize repository structure for better organization

- Move backend code to swingmusic/ folder
- Move client applications to root level (swingmusic-android, swingmusic-desktop, swingmusic-webclient)
- Remove intermediate backend/ and clients/ folders
- Update README with new folder structure and setup instructions
- Clean and organized repository layout

src/swingmusic/crons/mixes.py

@@ -1,25 +0,0 @@
-from swingmusic.crons.cron import CronJob
-from swingmusic.lib.recipes.artistmixes import ArtistMixes
-from swingmusic.lib.recipes.because import BecauseYouListened
-
-
-class Mixes(CronJob):
-    """
-    This cron job creates mixes displayed on the homepage.
-    """
-
-    name: str = "mixes"
-    hours: int = 12
-
-    def __init__(self):
-        super().__init__()
-
-    def run(self):
-        """
-        Creates the artist mixes
-        """
-        ArtistMixes()
-
-        # INFO: Because you listened to artist items are generated using
-        # the artist mixes, so run them after the artist mixes are created.
-        BecauseYouListened()