fix(web-app-mastodon): pin image to v4.6.2 instead of latest

mastodon/mastodon:latest floated to a Rails 8.1 major whose boot contract changed (fatal without a VAPID key), breaking the swarm deploy without any change on our side. Pin server and streaming to v4.6.2 so the boot contract can't shift under us again.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

roles/web-app-mastodon/meta/services.yml

@@ -41,7 +41,7 @@ mastodon:
   shared: true
   enabled: true
   image: ghcr.io/mastodon/mastodon
-  version: latest
+  version: v4.6.2
   min_storage: 15GB
   backup:
     no_stop_required: true
@@ -65,7 +65,7 @@ mastodon:
   lifecycle: beta
 streaming:
   image: ghcr.io/mastodon/mastodon-streaming
-  version: latest
+  version: v4.6.2
   name: mastodon-streaming
   cpus: "0.3"
   mem_reservation: 256m