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>

client/src/routes/public/FiveOnFriday.jsx

@@ -1,3 +1,4 @@
+import DOMPurify from 'dompurify'
 import PublicLayout from '../../components/PublicLayout.jsx'
 import PageHeader from '../../components/PageHeader.jsx'
 import { Loading, ErrorState, EmptyState } from '../../components/PageState.jsx'
@@ -36,7 +37,7 @@ export default function FiveOnFriday() {
                 {it.title}
               </h2>
               {it.body ? (
-                <div className="prose" dangerouslySetInnerHTML={{ __html: it.body }} />
+                <div className="prose" dangerouslySetInnerHTML={{ __html: DOMPurify.sanitize(it.body) }} />
               ) : (
                 it.excerpt && <p style={{ margin: 0, color: 'var(--text)' }}>{it.excerpt}</p>
               )}