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>

client/src/blocks/index.js

@@ -0,0 +1,19 @@
+// Client block registry entrypoint. Importing this module registers every
+// browser-side block definition (renderer + editor + palette entry) exactly
+// once, then re-exports the registry API. The page builder and the public page
+// renderer should import from HERE, not ./registry, so the definitions are
+// loaded before anything reads the registry.
+//
+// Wave 1 definitions are registered below as each block is built (spec build
+// order step 3), one import per block.
+
+export * from './registry'
+
+// ── Wave 1 block definitions ──────────────────────────────────────────
+// import './types/heading'    // added in step 3
+// import './types/richText'
+// import './types/image'
+// import './types/twoColumn'
+// import './types/cta'
+// import './types/divider'
+// import './types/quote'