Add pages table + block registry scaffold (page builder step 2)

New `pages` table: slug/title/blocks(JSON-as-text)/status/protected, author
FK, grouped SEO metadata + layout/nav settings columns (added up front per
spec — cheap now, painful to retrofit), published_at mirroring posts.

Block registry scaffold, server and client, defining the pattern without
any block types yet (Wave 1 lands in step 3):
- server/src/blocks: registry (register/get/list, reserved envelope keys,
  container metadata) + validateBlocks (authoritative save-time gate:
  envelope, registered-type, per-block schema, one-level nesting cap) +
  index entrypoint that will register Wave 1 defs.
- client/src/blocks: mirror registry carrying renderer/editor/palette +
  makeBlockId, plus index entrypoint.

Verified: schema applies idempotently against the dev DB (pages table +
indexes present); validator exercised for empty/non-array/unknown-type/
bad-envelope/duplicate-id/nested-container cases.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/db/schema.sql

@@ -486,6 +486,43 @@ CREATE TABLE IF NOT EXISTS mod_notes (
   INDEX idx_mod_notes_user (discord_user_id, created_at)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 
+-- Generic CMS pages composed from a fixed palette of blocks (the page builder).
+-- `blocks` is a JSON array of block-envelope objects ({ id, type, version,
+-- visible, props }); it is stored as text and parsed/validated in app code
+-- against the block registry (server/src/blocks) on every save — the same
+-- pattern role_menus.mapping uses, since MariaDB's JSON type is just LONGTEXT and
+-- the driver hands it back as a string anyway. The seo_*/og_image/canonical_url/
+-- robots and layout/nav_* columns are metadata/settings surfaced grouped in the
+-- API response; several have no consumer yet but are cheap to add now and painful
+-- to retrofit once real pages exist. published_at mirrors posts: stamped the first
+-- time a page goes to 'published'.
+CREATE TABLE IF NOT EXISTS pages (
+  id               INT AUTO_INCREMENT PRIMARY KEY,
+  slug             VARCHAR(160) NOT NULL UNIQUE,
+  title            VARCHAR(200) NOT NULL,
+  blocks           MEDIUMTEXT   NOT NULL,      -- JSON array of block objects
+  status           ENUM('draft','published') NOT NULL DEFAULT 'draft',
+  protected        TINYINT(1)   NOT NULL DEFAULT 0,
+  author_id        INT          NULL,
+  -- SEO / social metadata (grouped under `metadata` in the API response).
+  seo_title        VARCHAR(200) NULL,
+  meta_description VARCHAR(400) NULL,
+  og_image         VARCHAR(500) NULL,
+  canonical_url    VARCHAR(500) NULL,
+  robots           VARCHAR(100) NULL,
+  -- Presentation / navigation (grouped under `settings` in the API response).
+  layout           ENUM('default','full_width','landing') NOT NULL DEFAULT 'default',
+  show_in_nav      TINYINT(1)   NOT NULL DEFAULT 0,
+  nav_group        ENUM('main','footer','account','hidden') NULL,
+  nav_order        INT          NULL,
+  created_at       DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  updated_at       DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  published_at     DATETIME NULL,
+  CONSTRAINT fk_pages_author FOREIGN KEY (author_id) REFERENCES users(id) ON DELETE SET NULL,
+  INDEX idx_pages_status (status),
+  INDEX idx_pages_nav (show_in_nav, nav_group, nav_order)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
+
 -- Migrations for databases created before the wiki upgrade. Each statement uses
 -- IF NOT EXISTS so re-running on every boot is a harmless no-op. New installs get
 -- these columns from the CREATE TABLE above; existing installs get them here.