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/model/wiki/wiki.links.js

@@ -0,0 +1,15 @@
+// Extract internal wiki-link targets from a saved (already sanitized) body.
+// Internal links are anchors to /wiki/<slug> or elements carrying a
+// data-wiki-slug attribute. Returns a de-duplicated array of slugs.
+function extractTargets(html) {
+  if (!html) return []
+  const targets = new Set()
+  const hrefRe = /href="\/wiki\/([a-z0-9-]+)"/g
+  const dataRe = /data-wiki-slug="([a-z0-9-]+)"/g
+  let m
+  while ((m = hrefRe.exec(html))) targets.add(m[1])
+  while ((m = dataRe.exec(html))) targets.add(m[1])
+  return [...targets]
+}
+
+module.exports = { extractTargets }