feat(F1): notification content mapping and Web Push sender

Add vigilar/alerts/sender.py with _CONTENT_MAP for human-readable push
notification titles/bodies, build_notification(), and send_alert() which
retrieves VAPID key, iterates push subscriptions, calls pywebpush, and
logs results to alert_log with auto-cleanup of expired (410) endpoints.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/test_sender.py

@@ -0,0 +1,38 @@
+"""Tests for notification content builder and Web Push sender."""
+
+from vigilar.alerts.sender import build_notification
+from vigilar.constants import EventType, Severity
+
+
+def test_build_notification_person_detected():
+    result = build_notification(EventType.PERSON_DETECTED, Severity.WARNING, "front_entrance", {})
+    assert result["title"] == "Person Detected"
+    assert "Front Entrance" in result["body"]
+
+
+def test_build_notification_pet_escape():
+    result = build_notification(EventType.PET_ESCAPE, Severity.ALERT, "back_deck", {"pet_name": "Angel"})
+    assert result["title"] == "Pet Alert"
+    assert "Angel" in result["body"]
+
+
+def test_build_notification_wildlife_predator():
+    result = build_notification(EventType.WILDLIFE_PREDATOR, Severity.CRITICAL, "front_entrance", {"species": "bear"})
+    assert result["title"] == "Wildlife Alert"
+    assert "bear" in result["body"].lower()
+
+
+def test_build_notification_power_loss():
+    result = build_notification(EventType.POWER_LOSS, Severity.CRITICAL, "ups", {})
+    assert result["title"] == "Power Alert"
+    assert "battery" in result["body"].lower()
+
+
+def test_build_notification_unknown_event_type():
+    result = build_notification(EventType.MOTION_START, Severity.WARNING, "cam1", {})
+    assert result["title"] == "Vigilar Alert"
+
+
+def test_build_notification_has_url():
+    result = build_notification(EventType.PERSON_DETECTED, Severity.WARNING, "front", {})
+    assert "url" in result