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/migrations/base.py

@@ -1,12 +0,0 @@
-class Migration:
-    """
-    Base migration class.
-    """
-    enabled: bool = True
-
-    @staticmethod
-    def migrate():
-        """
-        Code to run when migrating, override this method.
-        """
-        pass