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/admin/admin.routes.js

@@ -90,6 +90,9 @@ adminRouter.put(
 )
 adminRouter.delete('/wiki/categories/:id', param('id').isInt(), validate, ctrl.deleteWikiCategory)
 
+// ── Wiki tags ──────────────────────────────────────────────────────────
+adminRouter.get('/wiki/tags', ctrl.listWikiTags)
+
 // ── Wiki pages ─────────────────────────────────────────────────────────
 adminRouter.get('/wiki', ctrl.listWiki)
 adminRouter.post(
@@ -99,6 +102,7 @@ adminRouter.post(
   body('excerpt').optional({ values: 'falsy' }).isString().isLength({ max: 400 }),
   body('category_id').optional({ values: 'null' }).isInt(),
   body('published').optional().isBoolean(),
+  body('tags').optional().isArray(),
   validate,
   ctrl.createWiki,
 )
@@ -109,6 +113,7 @@ adminRouter.put(
   body('excerpt').optional({ values: 'falsy' }).isString().isLength({ max: 400 }),
   body('category_id').optional({ values: 'null' }).isInt(),
   body('published').optional().isBoolean(),
+  body('tags').optional().isArray(),
   validate,
   ctrl.updateWiki,
 )