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/db/schema.sql

@@ -82,6 +82,22 @@ CREATE TABLE IF NOT EXISTS wiki_links (
   INDEX idx_wiki_links_target (target_slug)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
+-- Per-save content snapshots for history / diff / restore.
+CREATE TABLE IF NOT EXISTS wiki_revisions (
+  id          INT AUTO_INCREMENT PRIMARY KEY,
+  page_id     INT NOT NULL,
+  title       VARCHAR(200) NOT NULL,
+  body        MEDIUMTEXT NULL,
+  excerpt     VARCHAR(400) NULL,
+  category_id INT NULL,
+  editor_id   INT NULL,
+  change_note VARCHAR(280) NULL,
+  created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  CONSTRAINT fk_wiki_rev_page   FOREIGN KEY (page_id)   REFERENCES wiki_pages(id) ON DELETE CASCADE,
+  CONSTRAINT fk_wiki_rev_editor FOREIGN KEY (editor_id) REFERENCES users(id)      ON DELETE SET NULL,
+  INDEX idx_wiki_rev_page (page_id, id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
 CREATE TABLE IF NOT EXISTS settings (
   `key`      VARCHAR(64) PRIMARY KEY,
   value      TEXT NULL,