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/plugins/register.py

@@ -0,0 +1,18 @@
+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