Link page artifacts to preview route

mindspace-pages.mjs

@@ -1485,6 +1485,7 @@ async function registerPageArtifactForConversation({
       mimeType: contentMimeType,
       sizeBytes: contentBytes,
       storageKey,
+      canonicalUrl: `/api/mindspace/v1/pages/${encodeURIComponent(page.id)}/preview`,
       sortOrder: Number.isFinite(Number(versionNo)) ? Number(versionNo) : 0,
       now,
     });

mindspace-pages.test.mjs

@@ -181,6 +181,7 @@ test('registerPageArtifactForConversation records page artifact and refreshes ma
   assert.equal(calls[1][1].artifactKind, 'page');
   assert.equal(calls[1][1].messageId, 'message-1');
   assert.equal(calls[1][1].storageKey, 'users/user-1/pages/page-1/versions/v1.html');
+  assert.equal(calls[1][1].canonicalUrl, '/api/mindspace/v1/pages/page-1/preview');
   assert.deepEqual(calls[2], [
     'writeManifestForSession',
     { userId: 'user-1', sessionId: 'session-1' },