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/src/services/health_monitor.py

@@ -220,18 +220,18 @@ class HealthMonitor:
         await self._event_bus.publish(event_type, payload)
 
         # Create notification for instance owner (fire-and-forget)
+        # Only send warnings and errors; skip "recovered" info notifications.
         if new_status == "error":
             category = "instance"
             severity = "error"
             title = "Container failed"
-        else:
+        elif new_status == "unhealthy":
             category = "health"
-            if new_status == "unhealthy":
-                severity = "warning"
-                title = "Container unhealthy"
-            else:
-                severity = "info"
-                title = "Container recovered"
+            severity = "warning"
+            title = "Container unhealthy"
+        else:
+            # Running/recovered — do not notify
+            return
 
         try:
             await notification_service.create_notification(