fix: use append (>>) instead of overwrite (>) for cron logs and add missing dependencies

- Fixed api/schema.py to generate cron jobs using '>> wecker.log' so logs aren't truncated.
- Added test_bugfix_default_command_uses_append_for_logs in tests/test_api.py to prevent regression.
- Added RPi.GPIO and pygame to pyproject.toml via 'uv add' to fix ModuleNotFoundError in cron jobs.
- Updated README.md to reflect that system python packages are no longer needed.

api/schema.py

@@ -49,7 +49,7 @@ class Mutation:
         if command is None:
             project_root = Path(__file__).parent.parent.absolute()
             python_exec = sys.executable
-            command = f"cd {project_root} && {python_exec} wecker.py > wecker.log 2>&1"
+            command = f"cd {project_root} && {python_exec} wecker.py >> wecker.log 2>&1"
 
         manager = get_manager()
         new_id = manager.set_alarm(