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>

client/package-lock.json

@@ -12,6 +12,7 @@
         "@tiptap/extension-link": "^2.27.2",
         "@tiptap/react": "^2.27.2",
         "@tiptap/starter-kit": "^2.27.2",
+        "diff": "^5.2.2",
         "dompurify": "^3.4.11",
         "react": "^18.3.1",
         "react-dom": "^18.3.1",
@@ -1794,6 +1795,15 @@
         }
       }
     },
+    "node_modules/diff": {
+      "version": "5.2.2",
+      "resolved": "https://registry.npmjs.org/diff/-/diff-5.2.2.tgz",
+      "integrity": "sha512-vtcDfH3TOjP8UekytvnHH1o1P4FcUdt4eQ1Y+Abap1tk/OB2MWQvcwS2ClCd1zuIhc3JKOx6p3kod8Vfys3E+A==",
+      "license": "BSD-3-Clause",
+      "engines": {
+        "node": ">=0.3.1"
+      }
+    },
     "node_modules/dompurify": {
       "version": "3.4.11",
       "resolved": "https://registry.npmjs.org/dompurify/-/dompurify-3.4.11.tgz",