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/signal_alert.py

@@ -0,0 +1,30 @@
+"""signal_alert."""
+
+from __future__ import annotations
+
+import logging
+from os import getenv
+
+from apprise import Apprise
+
+logger = logging.getLogger(__name__)
+
+
+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)