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/plugins/register.py

@@ -1,18 +0,0 @@
-from swingmusic.db.userdata import PluginTable
-from sqlalchemy.exc import IntegrityError
-
-
-def register_plugins():
-    try:
-        PluginTable.insert_one(
-            {
-                "name": "lyrics_finder",
-                "active": False,
-                "settings": {"auto_download": False},
-                "extra": {
-                    "description": "Find lyrics from the internet",
-                },
-            }
-        )
-    except IntegrityError:
-        pass