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/embed_profile.html

@@ -152,8 +152,15 @@
           {% endfor %}
         </div>
         {% if let Some(comment) = entry.review().comment() %}
+        {% if entry.review().spoilers() %}
+        <details class="embed-comment spoiler">
+          <summary style="cursor:pointer;opacity:.75">Contains spoilers</summary>
+          {{ comment.value() }}
+        </details>
+        {% else %}
         <div class="embed-comment">{{ comment.value() }}</div>
         {% endif %}
+        {% endif %}
         <div class="embed-watched-at">{{ entry.review().watched_at().format("%Y-%m-%d") }}</div>
       </div>
     </article>