refactor: extract signal_alert into its own module

Move signal_alert out of python/common.py into a dedicated
python/signal_alert.py module and update its importers
(validate_system.py, snapshot_manager.py) to the new path.

Relocate the signal_alert tests from tests/test_common.py into
tests/test_signal_alert.py, repatching python.signal_alert.logger and
python.signal_alert.Apprise to match the new module.

python/common.py

@@ -4,11 +4,8 @@ from __future__ import annotations
 
 import logging
 from datetime import UTC, datetime
-from os import getenv
 from subprocess import PIPE, Popen
 
-from apprise import Apprise
-
 from python.logging_config import configure_logger as _configure_logger
 
 logger = logging.getLogger(__name__)
@@ -39,26 +36,6 @@ def bash_wrapper(command: str) -> tuple[str, int]:
     return output.decode(), process.returncode
 
 
-def signal_alert(body: str, title: str = "") -> None:
-    """Send a signal alert.
-
-    Args:
-        body (str): The body of the alert.
-        title (str, optional): The title of the alert. Defaults to "".
-    """
-    apprise_client = Apprise()
-
-    from_phone = getenv("SIGNAL_ALERT_FROM_PHONE")
-    to_phone = getenv("SIGNAL_ALERT_TO_PHONE")
-    if not from_phone or not to_phone:
-        logger.info("SIGNAL_ALERT_FROM_PHONE or SIGNAL_ALERT_TO_PHONE not set")
-        return
-
-    apprise_client.add(f"signal://localhost:8989/{from_phone}/{to_phone}")
-
-    apprise_client.notify(title=title, body=body)
-
-
 def utcnow() -> datetime:
     """Get the current UTC time."""
     return datetime.now(tz=UTC)