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/crons/cron.py

@@ -0,0 +1,23 @@
+import schedule
+
+
+from abc import ABC, abstractmethod
+
+
+class CronJob(ABC):
+    """
+    A cron job that will be run on a regular interval.
+    """
+
+    name: str
+    hours: int = 1
+
+    def __init__(self):
+        schedule.every(self.hours).hours.do(self.run)
+
+    @abstractmethod
+    def run(self):
+        """
+        The function that will be called by the cron job.
+        """
+        ...