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/models/lastfm.py

@@ -0,0 +1,28 @@
+from dataclasses import dataclass
+from typing import Any
+
+
+@dataclass
+class SimilarArtistEntry:
+    artisthash: str
+    name: str
+    weight: float
+    scrobbles: int
+    listeners: int
+
+
+@dataclass
+class SimilarArtist:
+    artisthash: str
+    similar_artists: list[SimilarArtistEntry]
+
+
+    def get_artist_hash_set(self) -> set[str]:
+        """
+        Returns a set of similar artists.
+        """
+        if not self.similar_artists:
+            return set()
+
+        # INFO: 
+        return set(a['artisthash'] for a in self.similar_artists)