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/src/blocks/registry.js

@@ -0,0 +1,96 @@
+// Block registry (server side) — the single source of truth for what block
+// types exist, how their props validate, and how long a rendered block may be
+// cached. The admin builder UI, the public renderer, and this server-side
+// validation are all driven from a registry entry rather than a switch statement
+// scattered across files: adding a block later means adding ONE entry (here on
+// the server for schema/cache, and one in client/src/blocks for the React
+// renderer/editor), not editing four places.
+//
+// A registered definition looks like:
+//   {
+//     type: 'heading',        // stable string id, unique across the registry
+//     version: 1,             // prop-schema version; bump when props change so a
+//                             // one-time migration can transform older blocks
+//     schema: (props) => [],  // returns an array of error strings ([] = valid)
+//     cacheTTL: null,         // seconds a rendered instance may be cached;
+//                             // null = never cache (static blocks). Dynamic
+//                             // Wave 2 blocks set this (e.g. server_status: 10).
+//     container: false,       // true only for block types that hold sub-blocks
+//     containerSlots: [],     // prop keys holding sub-block arrays, e.g.
+//                             // ['left','right'] for two_column
+//   }
+//
+// This module is intentionally empty of block types — it only defines the
+// pattern. Wave 1 block definitions register themselves via ./index.js.
+
+// The only keys allowed at the top level of a stored block object. Everything
+// block-specific lives inside `props`; nothing else lives at the top level.
+// Ordering is the array position, not a stored field — so a reorder is just a
+// reorder of the array, and `id` is never derived from position.
+const RESERVED_KEYS = Object.freeze(['id', 'type', 'version', 'visible', 'props'])
+
+const registry = new Map()
+
+/**
+ * Register a block definition. Throws on a missing type or a duplicate — both
+ * are programmer errors surfaced at boot, not runtime input.
+ * @param {object} def
+ * @returns {object} the normalized, frozen definition
+ */
+function registerBlock(def) {
+  if (!def || typeof def.type !== 'string' || def.type.length === 0) {
+    throw new Error('registerBlock: a block definition needs a string `type`')
+  }
+  if (registry.has(def.type)) {
+    throw new Error(`registerBlock: block type already registered: ${def.type}`)
+  }
+  if (def.schema != null && typeof def.schema !== 'function') {
+    throw new Error(`registerBlock: ${def.type}.schema must be a function`)
+  }
+  const containerSlots = def.containerSlots || []
+  if (def.container && containerSlots.length === 0) {
+    throw new Error(`registerBlock: container block ${def.type} needs containerSlots`)
+  }
+  const entry = Object.freeze({
+    type: def.type,
+    version: Number.isInteger(def.version) ? def.version : 1,
+    schema: def.schema || null,
+    cacheTTL: def.cacheTTL == null ? null : Number(def.cacheTTL),
+    container: Boolean(def.container),
+    containerSlots: Object.freeze([...containerSlots]),
+  })
+  registry.set(entry.type, entry)
+  return entry
+}
+
+/** @returns {object|null} the definition for `type`, or null if unknown. */
+function getBlock(type) {
+  return registry.get(type) || null
+}
+
+/** @returns {boolean} whether `type` is a registered block. */
+function hasBlock(type) {
+  return registry.has(type)
+}
+
+/** @returns {object[]} all registered definitions (registration order). */
+function listBlocks() {
+  return [...registry.values()]
+}
+
+/**
+ * Drop every registered block. Test-only — lets a suite register a fixture set
+ * and start from a known-empty registry.
+ */
+function _resetRegistry() {
+  registry.clear()
+}
+
+module.exports = {
+  RESERVED_KEYS,
+  registerBlock,
+  getBlock,
+  hasBlock,
+  listBlocks,
+  _resetRegistry,
+}