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>

WIKI_UPGRADE.md

@@ -309,10 +309,17 @@ phase if preferred). Do not merge a phase that hasn't been verified.
   `<script>` stripped); a toolbar edit (insert divider) saved and persisted. TipTap
   is code-split into its own chunk (lazy-loaded), keeping it off the public bundle.
 
-### Phase 3 — Connectivity
+### Phase 3 — Connectivity ✅
 - Internal `[[slug]]` links + red-link detection; `wiki_links` rebuild on save;
   backlinks on the article; tags + tag/category filtering.
 - **Exit check**: link page A→B, confirm B shows A under "Linked from"; tag filter works.
+- **Verified** (2026-06-27): internal links authored via an in-editor page picker
+  (links to `/wiki/<slug>`); A→B made B list A under "Linked from"; a link to a
+  non-existent page renders as a red link; removing the link on save cleared the
+  backlink (link index rebuilt). Tags upsert on save, filter via `?tag=` (chips +
+  flat index view), list with published counts, and orphan tags are auto-pruned.
+- Implementation note: links are plain anchors to `/wiki/<slug>` (the WYSIWYG fits
+  this better than `[[ ]]` syntax); the sanitizer also allows `data-wiki-slug`.
 
 ### Phase 4 — Discovery & trust
 - FULLTEXT search (public search box + admin filter); revision history list /