feat: filter notifications to warnings/errors/ready only and add clear-all button

Notification filtering:
- lifecycle_hooks.py: only instance.error and instance.health_changed
  with status=running generate notifications. All other lifecycle events
  (created, started, stopped, restarted, deleted) are filtered out.
- health_monitor.py: only error and unhealthy states generate notifications.
  Running/recovered state no longer creates info notifications.
- _derive_title now maps instance.health_changed to "Container ready".

Clear-all button:
- Added dismiss_all() to NotificationService
- Added DELETE /notifications endpoint for bulk dismiss
- Frontend: clearAllNotifications API, clearAll in notification context,
  "Clear all" button in notification drawer alongside "Mark all as read"
- Added CSS for .notification-clear-all with danger hover state
- Updated notification-center tests

Quality gates: pytest (21 passed), vitest (11 passed)

apps/api/tests/unit/test_lifecycle_hooks.py

@@ -0,0 +1,49 @@
+"""Unit tests for lifecycle hook helpers."""
+
+import pytest
+
+from src.services.lifecycle_hooks import _derive_title, _should_notify
+
+
+class TestDeriveTitle:
+    """Tests for _derive_title."""
+
+    def test_known_event_types(self) -> None:
+        assert _derive_title("instance.created") == "Container created"
+        assert _derive_title("instance.started") == "Container started"
+        assert _derive_title("instance.stopped") == "Container stopped"
+        assert _derive_title("instance.restarted") == "Container restarted"
+        assert _derive_title("instance.deleted") == "Container deleted"
+        assert _derive_title("instance.error") == "Container error"
+        assert _derive_title("instance.health_changed") == "Container ready"
+
+    def test_unknown_event_type(self) -> None:
+        assert _derive_title("instance.custom_event") == "Custom Event"
+
+
+class TestShouldNotify:
+    """Tests for _should_notify filtering."""
+
+    def test_error_events_are_notified(self) -> None:
+        assert _should_notify("instance.error", "error") is True
+        assert _should_notify("instance.error", None) is True
+
+    def test_health_changed_running_is_notified(self) -> None:
+        assert _should_notify("instance.health_changed", "running") is True
+
+    def test_created_started_stopped_restarted_deleted_filtered(self) -> None:
+        for event in [
+            "instance.created",
+            "instance.started",
+            "instance.stopped",
+            "instance.restarted",
+            "instance.deleted",
+        ]:
+            assert _should_notify(event, "pending") is False
+            assert _should_notify(event, "running") is False
+            assert _should_notify(event, None) is False
+
+    def test_health_changed_non_running_filtered(self) -> None:
+        assert _should_notify("instance.health_changed", "unhealthy") is False
+        assert _should_notify("instance.health_changed", "starting") is False
+        assert _should_notify("instance.health_changed", None) is False