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/db/schema.sql

@@ -58,6 +58,30 @@ CREATE TABLE IF NOT EXISTS wiki_pages (
   FULLTEXT INDEX idx_wiki_search (title, body)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
+-- Wiki tags (many-to-many with pages).
+CREATE TABLE IF NOT EXISTS wiki_tags (
+  id    INT AUTO_INCREMENT PRIMARY KEY,
+  slug  VARCHAR(120) NOT NULL UNIQUE,
+  label VARCHAR(120) NOT NULL
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+CREATE TABLE IF NOT EXISTS wiki_page_tags (
+  page_id INT NOT NULL,
+  tag_id  INT NOT NULL,
+  PRIMARY KEY (page_id, tag_id),
+  CONSTRAINT fk_wpt_page FOREIGN KEY (page_id) REFERENCES wiki_pages(id) ON DELETE CASCADE,
+  CONSTRAINT fk_wpt_tag  FOREIGN KEY (tag_id)  REFERENCES wiki_tags(id)  ON DELETE CASCADE
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
+-- Internal-link index, rebuilt on each save. target_slug may point at a page
+-- that does not exist yet (a "red link").
+CREATE TABLE IF NOT EXISTS wiki_links (
+  source_page_id INT NOT NULL,
+  target_slug    VARCHAR(120) NOT NULL,
+  CONSTRAINT fk_wiki_links_src FOREIGN KEY (source_page_id) REFERENCES wiki_pages(id) ON DELETE CASCADE,
+  INDEX idx_wiki_links_target (target_slug)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
 CREATE TABLE IF NOT EXISTS settings (
   `key`      VARCHAR(64) PRIMARY KEY,
   value      TEXT NULL,