Wiki Phase 3: internal links, backlinks, and tags

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

Schema (additive new tables): wiki_tags, wiki_page_tags, wiki_links.

Internal links & backlinks:
- new wiki.links.js parses a saved body for /wiki/<slug> (and data-wiki-slug)
  targets; wiki_links is rebuilt on every save
- article shows a "Linked from" section (published backlinks) and renders
  links to non-existent pages as red links (server returns missing_links)
- editor gains an internal-link picker listing existing pages

Tags:
- pages accept a tags[] array; tags upsert on save, page tag-set is replaced,
  and orphaned tags are auto-pruned (on save and delete)
- public/admin list filter by ?tag=; /wiki/tags lists tags with published counts
- article shows tag chips; the index has a flat tag-filtered view; editor has a
  comma-separated tags field

Verified end-to-end: A->B backlink appears, red link detected, link index
rebuilds on edit, tag filtering + chips + pruning all work.

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

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

@@ -25,8 +25,9 @@ publicRouter.post(
 publicRouter.get('/posts/:category', siteMode, ctrl.getPosts)
 publicRouter.get('/posts/:category/:idOrSlug', siteMode, ctrl.getPost)
 publicRouter.get('/wiki', siteMode, ctrl.getWikiList)
-// Static path must precede the :slug route so it isn't captured as a slug.
+// Static paths must precede the :slug route so they aren't captured as a slug.
 publicRouter.get('/wiki/categories', siteMode, ctrl.getWikiCategories)
+publicRouter.get('/wiki/tags', siteMode, ctrl.getWikiTags)
 publicRouter.get('/wiki/:slug', siteMode, ctrl.getWikiPage)
 
 module.exports = publicRouter