fix: restore public page share thumbnails from SVG sidecars

Serve missing *.thumbnail.png requests when the SVG sidecar exists, and schedule async PNG rasterization after public HTML is materialized so WeChat og:image URLs stop 404ing.

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

mindspace-public-route.test.mjs

@@ -120,3 +120,30 @@ test('resolveMindSpacePublicRequest coordinates canonical redirect, serve, and n
     ownerKey,
   });
 });
+
+test('resolveMindSpacePublicRequest serves missing thumbnail png when svg sidecar exists', async () => {
+  const root = fs.mkdtempSync(path.join(os.tmpdir(), 'ms-public-thumb-'));
+  const ownerKey = '123e4567-e89b-12d3-a456-426614174000';
+  const targetDir = path.join(root, 'MindSpace', ownerKey);
+  const publicDir = path.join(targetDir, 'public');
+  fs.mkdirSync(publicDir, { recursive: true });
+  fs.writeFileSync(path.join(publicDir, 'report.thumbnail.svg'), '<svg xmlns="http://www.w3.org/2000/svg"></svg>');
+
+  const served = await resolveMindSpacePublicRequest({
+    h5Root: root,
+    requestPath: `/${ownerKey}/public/report.thumbnail.png`,
+  });
+  assert.equal(served.action, 'serve');
+  assert.equal(served.filePath, path.join(publicDir, 'report.thumbnail.png'));
+  assert.equal(fs.existsSync(served.filePath), false);
+
+  const stillMissing = await resolveMindSpacePublicRequest({
+    h5Root: root,
+    requestPath: `/${ownerKey}/public/missing.thumbnail.png`,
+  });
+  assert.deepEqual(stillMissing, {
+    action: 'not_found',
+    reason: 'missing_file',
+    ownerKey,
+  });
+});