feat(page-data): add private and public dataset API (phase 1-3)

Extract UserDataSpaceService for shared SQLite access, wire logged-in Page
Data routes, and add public insert plus password-token read/update/delete with
policy storage, rate limits, and regression tests.

Co-authored-by: Cursor <cursoragent@cursor.com>

page-access-policy.test.mjs

@@ -0,0 +1,67 @@
+import assert from 'node:assert/strict';
+import test from 'node:test';
+import {
+  buildEffectiveDataset,
+  normalizePageAccessPolicy,
+  policyAllowsAction,
+} from './page-access-policy.mjs';
+
+test('normalizePageAccessPolicy validates access mode and datasets', () => {
+  const policy = normalizePageAccessPolicy(
+    {
+      pageId: 'page-1',
+      ownerUserId: 'user-1',
+      accessMode: 'public',
+      datasets: {
+        registrations: {
+          insert: true,
+          columns: { insert: ['name', 'phone'] },
+        },
+      },
+    },
+    { fallbackPageId: 'page-1', fallbackOwnerUserId: 'user-1' },
+  );
+  assert.equal(policy.accessMode, 'public');
+  assert.equal(policy.datasets.registrations.insert, true);
+  assert.equal(policy.datasets.registrations.read, false);
+});
+
+test('buildEffectiveDataset intersects registry actions with policy permissions', () => {
+  const effective = buildEffectiveDataset(
+    {
+      name: 'registrations',
+      table: 'form_registrations',
+      description: '报名',
+      actions: ['read', 'insert', 'update', 'soft_delete'],
+      columns: {
+        read: ['id', 'name', 'phone', 'secret'],
+        insert: ['name', 'phone', 'secret'],
+        update: ['status'],
+        soft_delete: ['id'],
+      },
+    },
+    {
+      read: false,
+      insert: true,
+      update: false,
+      softDelete: false,
+      columns: { insert: ['name', 'phone'] },
+    },
+  );
+  assert.deepEqual(effective.actions, ['insert']);
+  assert.deepEqual(effective.columns.insert, ['name', 'phone']);
+});
+
+test('policyAllowsAction respects per-dataset flags', () => {
+  const policy = normalizePageAccessPolicy({
+    pageId: 'page-1',
+    ownerUserId: 'user-1',
+    accessMode: 'password',
+    datasets: {
+      ledger: { read: true, insert: true, update: true, softDelete: true, columns: {} },
+    },
+  });
+  assert.equal(policyAllowsAction(policy, 'ledger', 'read'), true);
+  assert.equal(policyAllowsAction(policy, 'ledger', 'insert'), true);
+  assert.equal(policyAllowsAction(policy, 'missing', 'read'), false);
+});