Move backend files to root level for cleaner GitHub display

- Move all backend files from swingmusic/ to root level
- Backend files now display directly on GitHub repository page
- Keep client applications as submodules (swingmusic-android, swingmusic-desktop, swingmusic-webclient)
- Update README to reflect new structure (no cd swingmusic needed)
- Cleaner, more professional GitHub repository layout

Files moved to root:
- src/ (main source code)
- pyproject.toml, requirements.txt, run.py
- swingmusic.spec, uv.lock, version.txt
- services/

Result: GitHub shows backend files directly while maintaining organized structure

src/swingmusic/crons/mixes.py

@@ -0,0 +1,25 @@
+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()