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>

client/src/api/client.js

@@ -49,8 +49,15 @@ export const api = {
   status: () => req('/public/status'),
   posts: (category) => req(`/public/posts/${category}`),
   post: (category, idOrSlug) => req(`/public/posts/${category}/${idOrSlug}`),
-  wiki: (category) => req(`/public/wiki${category ? `?category=${encodeURIComponent(category)}` : ''}`),
+  wiki: (opts = {}) => {
+    const qs = new URLSearchParams()
+    if (opts.category) qs.set('category', opts.category)
+    if (opts.tag) qs.set('tag', opts.tag)
+    const s = qs.toString()
+    return req(`/public/wiki${s ? `?${s}` : ''}`)
+  },
   wikiCategories: () => req('/public/wiki/categories'),
+  wikiTags: () => req('/public/wiki/tags'),
   wikiPage: (slug) => req(`/public/wiki/${slug}`),
   contact: (payload) => req('/public/contact', { method: 'POST', body: payload }),
 
@@ -83,6 +90,7 @@ export const api = {
     publishWiki: (slug, published) =>
       req(`/admin/wiki/${slug}/publish`, { method: 'PATCH', body: { published } }),
     deleteWiki: (slug) => req(`/admin/wiki/${slug}`, { method: 'DELETE' }),
+    listWikiTags: () => req('/admin/wiki/tags'),
     listWikiCategories: () => req('/admin/wiki/categories'),
     createWikiCategory: (data) => req('/admin/wiki/categories', { method: 'POST', body: data }),
     updateWikiCategory: (id, data) =>