RTE Posts upgrade: TipTap editor + sanitization for posts

Extend the wiki's RichTextEditor to the Posts editor and close the
stored-XSS gap on public post bodies.

- RichTextEditor: add `variant` prop — `full` (wiki), `post` (no
  internal wiki-page link picker), `minimal` (image-only, for
  Screenshots captions). Toolbar sections rendered conditionally.
- PostEditor: replace the body textarea with a lazy-loaded
  RichTextEditor in Suspense; variant chosen by category
  (minimal for screenshots, post otherwise).
- posts.model: sanitize body via shared cleanBody on create/update,
  treat an empty TipTap `<p></p>` as null, and auto-derive the
  excerpt from the body (max 280 chars) when left blank.
- sanitizeHtml util: add deriveExcerpt() helper.
- FiveOnFriday / NewsletterIssue: wrap dangerouslySetInnerHTML with
  DOMPurify.sanitize() as defense-in-depth on render.

No schema or dependency changes. Verified end-to-end against the
local stack: 24/24 API assertions and a full UI round-trip across
all four post categories.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/utils/sanitizeHtml.js

@@ -42,4 +42,22 @@ function cleanBody(html) {
   return sanitizeHtml(String(html), OPTIONS)
 }
 
-module.exports = { cleanBody, OPTIONS }
+/**
+ * Derive a plain-text excerpt from body HTML. Strips tags, collapses
+ * whitespace, and truncates to `max` chars (with an ellipsis). Used as the
+ * excerpt fallback when an author leaves the excerpt field blank.
+ * @param {string|null|undefined} html
+ * @param {number} [max=280]
+ * @returns {string|null}
+ */
+function deriveExcerpt(html, max = 280) {
+  if (html == null) return null
+  const text = String(html)
+    .replace(/<[^>]+>/g, ' ')
+    .replace(/\s+/g, ' ')
+    .trim()
+  if (!text) return null
+  return text.length > max ? `${text.slice(0, max - 3)}...` : text
+}
+
+module.exports = { cleanBody, deriveExcerpt, OPTIONS }