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/NewsletterIssue.jsx

@@ -1,4 +1,5 @@
 import { Link, useParams } from 'react-router-dom'
+import DOMPurify from 'dompurify'
 import PublicLayout from '../../components/PublicLayout.jsx'
 import { Loading, ErrorState } from '../../components/PageState.jsx'
 import { useAsync } from '../../lib/useAsync.js'
@@ -52,7 +53,7 @@ function Issue({ issue }) {
       {issue.excerpt && <p style={{ margin: '14px 0 0', color: 'var(--muted)', fontSize: '1.1rem' }}>{issue.excerpt}</p>}
       <div style={{ height: 1, background: 'var(--line)', margin: '28px 0' }} />
       {issue.body ? (
-        <div className="prose" dangerouslySetInnerHTML={{ __html: issue.body }} />
+        <div className="prose" dangerouslySetInnerHTML={{ __html: DOMPurify.sanitize(issue.body) }} />
       ) : (
         <p className="muted">This issue has no content yet.</p>
       )}