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/routes/wiki/WikiArticle.jsx

@@ -15,8 +15,9 @@ function slugify(text) {
 }
 
 // Parse the stored body HTML: sanitize (defense in depth — the server also
-// sanitizes on save), then assign ids to <h2> headings and collect a TOC.
-function buildArticle(body) {
+// sanitizes on save), assign ids to <h2> headings and collect a TOC, and mark
+// internal links to pages that don't exist as "red links".
+function buildArticle(body, missing) {
   if (!body) return { html: '', toc: [] }
   if (typeof window === 'undefined' || !window.DOMParser) return { html: '', toc: [] }
   const safe = DOMPurify.sanitize(body)
@@ -27,13 +28,22 @@ function buildArticle(body) {
     h.id = id
     toc.push({ id, label: h.textContent })
   })
+  doc.querySelectorAll('a[href^="/wiki/"]').forEach((a) => {
+    const target = a.getAttribute('href').replace(/^\/wiki\//, '').replace(/[#?].*$/, '')
+    a.removeAttribute('target') // internal links stay in-app
+    if (missing.has(target)) {
+      a.classList.add('wiki-red-link')
+      a.setAttribute('title', 'This page does not exist yet')
+    }
+  })
   return { html: doc.body.innerHTML, toc }
 }
 
 export default function WikiArticle() {
   const { slug } = useParams()
   const { loading, error, data: page } = useAsync(() => api.wikiPage(slug), [slug])
-  const { html, toc } = useMemo(() => buildArticle(page?.body), [page])
+  const missing = useMemo(() => new Set(page?.missing_links || []), [page])
+  const { html, toc } = useMemo(() => buildArticle(page?.body, missing), [page, missing])
 
   return (
     <PublicLayout section="wiki">
@@ -100,6 +110,15 @@ export default function WikiArticle() {
               <p className="sans" style={{ margin: '18px 0 0', color: 'var(--dim)', fontSize: '0.78rem', letterSpacing: '0.04em' }}>
                 Last updated {longDate(page.updated_at) || '—'}
               </p>
+              {page.tags && page.tags.length > 0 && (
+                <div style={{ display: 'flex', flexWrap: 'wrap', gap: 8, marginTop: 14 }}>
+                  {page.tags.map((t) => (
+                    <Link key={t.slug} to={`/wiki?tag=${t.slug}`} className="wiki-tag">
+                      #{t.label}
+                    </Link>
+                  ))}
+                </div>
+              )}
               <div style={{ height: 1, background: 'var(--line)', margin: '30px 0' }} />
               {html ? (
                 <div className="prose" dangerouslySetInnerHTML={{ __html: html }} />
@@ -107,6 +126,28 @@ export default function WikiArticle() {
                 <p className="muted">This page has no content yet.</p>
               )}
 
+              {page.backlinks && page.backlinks.length > 0 && (
+                <section
+                  style={{ marginTop: 40, borderTop: '1px solid var(--line)', paddingTop: 22 }}
+                >
+                  <p
+                    className="sans"
+                    style={{ margin: '0 0 12px', color: 'var(--accent)', fontSize: '0.66rem', fontWeight: 700, letterSpacing: '0.14em', textTransform: 'uppercase' }}
+                  >
+                    Linked from
+                  </p>
+                  <ul style={{ margin: 0, paddingLeft: 18, fontFamily: 'var(--sans)', fontSize: '0.92rem' }}>
+                    {page.backlinks.map((b) => (
+                      <li key={b.slug} style={{ marginBottom: 6 }}>
+                        <Link to={`/wiki/${b.slug}`} style={{ color: 'var(--accent)', textDecoration: 'none' }}>
+                          {b.title}
+                        </Link>
+                      </li>
+                    ))}
+                  </ul>
+                </section>
+              )}
+
               <nav style={{ display: 'flex', justifyContent: 'flex-start', marginTop: 40 }}>
                 <Link to="/wiki" className="pill">
                   ← All wiki pages