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/crons/cron.py

@@ -1,23 +0,0 @@
-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.
-        """
-        ...