feat: mentions and following tickets

Mention project members in comments with @, follow and unfollow tickets,
and get told when something changes on the tickets you follow. Commenting
on a ticket starts following it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/web/tracker/infrastructure/src/lib/graphql/fragments.graphql

@@ -88,6 +88,9 @@ fragment IssueFields on Issue {
     issueId
     issueKey
   }
+  watchers {
+    ...UserRefFields
+  }
   attachments {
     id
     fileName