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/activity_feed.html

@@ -45,9 +45,17 @@
       {% if let Some(wm) = entry.review().watch_medium() %}
       <span class="watch-medium">{{ wm.label() }}</span>
       {% endif %}
+      {% let author = ctx.is_current_user(entry.review().user_id().value()) %}
       {% if let Some(comment) = entry.review().comment() %}
+      {% if !author && entry.review().spoilers() %}
+      <details class="comment spoiler">
+        <summary style="cursor:pointer;opacity:.75">Contains spoilers</summary>
+        {{ comment.value() }}
+      </details>
+      {% else %}
       <div class="comment">{{ comment.value() }}</div>
       {% endif %}
+      {% endif %}
       <div class="feed-meta">
         {% match entry.review().source() %}
         {% when ReviewSource::Remote with { actor_url } %}