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/migrations/base.py

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