Add Spoiler Flag with veil, AP content-warning encoding, and full-stack plumbing

Authors can flag a review's comment as containing spoilers; the comment is
blurred behind a reveal control for every viewer except the author, omitted
from RSS, and federates as containsSpoilers plus an AP summary content
warning (ADR-0011) so Mastodon-class clients also collapse it. Flag follows
the general-edit path (ADR-0001) across domain, both DB backends, REST,
HTML templates, SPA (shadcn), TUI fixtures, and exports.

crates/adapters/template-askama/templates/new_review.html

@@ -35,6 +35,9 @@
     Comment<br>
     <textarea name="comment"></textarea>
   </label>
+  <label>
+    Contains spoilers <input type="checkbox" name="spoilers" value="true">
+  </label>
   <label>
     Watched via <span class="optional">(optional)</span><br>
     <select name="watch_medium">