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_notification_service.py

@@ -308,6 +308,55 @@ async def test_get_unread_count_excludes_dismissed(
     assert count == 0
 
 
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_dismiss_all_affects_all_non_dismissed(
+    db_session: AsyncSession,
+    notification_service: NotificationService,
+    user_a: User,
+) -> None:
+    for i in range(4):
+        await notification_service.create_notification(
+            db_session,
+            user_a.id,
+            category="instance",
+            severity="info",
+            title=f"Notification {i}",
+        )
+
+    cleared = await notification_service.dismiss_all(db_session, user_a.id)
+
+    assert cleared == 4
+    items, total = await notification_service.list_notifications(db_session, user_a.id)
+    assert total == 0
+
+
+@pytest.mark.unit
+@pytest.mark.asyncio
+async def test_dismiss_all_affects_only_caller(
+    db_session: AsyncSession,
+    notification_service: NotificationService,
+    user_a: User,
+    user_b: User,
+) -> None:
+    for i in range(3):
+        await notification_service.create_notification(
+            db_session, user_a.id, category="instance", severity="info", title=f"A-{i}"
+        )
+    for i in range(2):
+        await notification_service.create_notification(
+            db_session, user_b.id, category="instance", severity="info", title=f"B-{i}"
+        )
+
+    cleared = await notification_service.dismiss_all(db_session, user_a.id)
+
+    assert cleared == 3
+    items_a, total_a = await notification_service.list_notifications(db_session, user_a.id)
+    items_b, total_b = await notification_service.list_notifications(db_session, user_b.id)
+    assert total_a == 0
+    assert total_b == 2
+
+
 @pytest.mark.unit
 @pytest.mark.asyncio
 async def test_mark_all_read_affects_only_caller(