Wiki Phase 4: full-text search + revision history

Final phase of the wiki upgrade (see WIKI_UPGRADE.md).

Schema (additive): wiki_revisions table (per-save content snapshots).
The FULLTEXT index on wiki_pages(title, body) shipped in Phase 1.

Search:
- MATCH ... AGAINST natural-language search over title + body, ordered by
  relevance
- public: GET /public/wiki?q= (published only); admin: GET /admin/wiki?q=
  (all statuses)
- public wiki index gains a search box; admin list gains a search field

Revision history:
- every create/update snapshots the page into wiki_revisions
- admin endpoints: list revisions, get one, and restore (restore overwrites
  the page, rebuilds links, and appends a new revision — history stays
  append-only); logged as wiki.revision.restore
- editor gains a History modal: revision list + word-level diff (jsdiff) of a
  chosen revision against the current page, with one-click restore

Verified end-to-end: search matches body and title; two edits produce three
revisions; diff renders added/removed words; restore reverts and records a new
revision. No console errors.

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

server/src/router/v1/admin/admin.routes.js

@@ -114,6 +114,7 @@ adminRouter.put(
   body('category_id').optional({ values: 'null' }).isInt(),
   body('published').optional().isBoolean(),
   body('tags').optional().isArray(),
+  body('change_note').optional({ values: 'falsy' }).isString().isLength({ max: 280 }),
   validate,
   ctrl.updateWiki,
 )
@@ -123,6 +124,14 @@ adminRouter.patch(
   validate,
   ctrl.publishWiki,
 )
+adminRouter.get('/wiki/:slug/revisions', ctrl.listWikiRevisions)
+adminRouter.get('/wiki/:slug/revisions/:id', param('id').isInt(), validate, ctrl.getWikiRevision)
+adminRouter.post(
+  '/wiki/:slug/revisions/:id/restore',
+  param('id').isInt(),
+  validate,
+  ctrl.restoreWikiRevision,
+)
 adminRouter.delete('/wiki/:slug', ctrl.deleteWiki)
 
 // ── Settings ──────────────────────────────────────────────────────────