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/styles/theme.css

@@ -314,10 +314,10 @@ button[disabled] {
 
 /* ===== Rich text editor (TipTap) ===== */
 .rte {
+  position: relative;
   border: 1px solid var(--line);
   border-radius: 8px;
   background: var(--bg);
-  overflow: hidden;
 }
 .rte:focus-within {
   border-color: var(--accent);
@@ -380,6 +380,72 @@ button[disabled] {
   color: var(--dim);
   pointer-events: none;
 }
+/* Internal-link picker popover */
+.rte-linkmenu {
+  position: absolute;
+  z-index: 20;
+  top: 50px;
+  left: 10px;
+  width: min(360px, calc(100% - 20px));
+  padding: 10px;
+  border: 1px solid var(--line);
+  border-radius: 8px;
+  background: var(--panel-a);
+  box-shadow: var(--shadow-card);
+}
+.rte-linkmenu-list {
+  margin-top: 8px;
+  max-height: 220px;
+  overflow-y: auto;
+  display: flex;
+  flex-direction: column;
+  gap: 2px;
+}
+.rte-linkmenu-item {
+  display: flex;
+  justify-content: space-between;
+  align-items: baseline;
+  gap: 12px;
+  padding: 7px 9px;
+  border: none;
+  border-radius: 6px;
+  background: transparent;
+  color: var(--text);
+  font-family: var(--sans);
+  font-size: 0.88rem;
+  text-align: left;
+  cursor: pointer;
+}
+.rte-linkmenu-item:hover {
+  background: var(--blue);
+  color: var(--ink);
+}
+.rte-linkmenu-slug {
+  color: var(--dim);
+  font-family: ui-monospace, Menlo, monospace;
+  font-size: 0.76rem;
+}
+
+/* ===== Wiki connectivity (tags + red links) ===== */
+.wiki-tag {
+  display: inline-block;
+  padding: 3px 10px;
+  border: 1px solid var(--line);
+  border-radius: 999px;
+  background: rgba(127, 153, 189, 0.1);
+  color: var(--accent);
+  font-family: var(--sans);
+  font-size: 0.78rem;
+  text-decoration: none;
+}
+.wiki-tag:hover {
+  border-color: var(--accent);
+  background: var(--blue);
+}
+.prose a.wiki-red-link {
+  color: #d98b84;
+  border-bottom: 1px dotted #d98b84;
+}
 
 /* ===== Admin tables ===== */
 .adm-table {