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/lib/recipes/because.py

@@ -0,0 +1,40 @@
+from pprint import pprint
+from swingmusic.db.userdata import UserTable
+from swingmusic.lib.recipes import HomepageRoutine
+from swingmusic.lib.recipes.artistmixes import ArtistMixes
+from swingmusic.models.mix import Mix
+from swingmusic.plugins.mixes import MixesPlugin
+from swingmusic.store.homepage import HomepageStore
+
+
+class BecauseYouListened(HomepageRoutine):
+    store_keys = ["because_you_listened_to_artist", "artists_you_might_like"]
+
+    @property
+    def is_valid(self):
+        return MixesPlugin().enabled
+
+    def run(self):
+        users = UserTable.get_all()
+
+        for user in users:
+            entry: dict[str, Mix] = HomepageStore.entries.get(
+                ArtistMixes.store_key
+            ).items.get(user.id)  # type: ignore
+
+            if not entry:
+                continue
+
+            because_you_listened_to_artist, artists_you_might_like = (
+                MixesPlugin().get_because_items(list(entry.values()))
+            )
+
+            if not because_you_listened_to_artist or not artists_you_might_like:
+                continue
+
+            HomepageStore.entries[self.store_keys[0]].items[
+                user.id
+            ] = because_you_listened_to_artist
+            HomepageStore.entries[self.store_keys[1]].items[
+                user.id
+            ] = artists_you_might_like