fix: prevent metadata fetch from overwriting fallback thumbnail with null

Use logical OR when merging fetched metadata so null values from
failed og: extraction don't destroy valid defaults (e.g. YouTube
thumbnail set before the fetch).

packages/server/src/utils/embedResolver.ts

@@ -102,9 +102,9 @@ export async function resolveEmbeds(
               effectiveEmbedType = 'audio';
             }
           } else {
-            title = metadata.title;
-            description = metadata.description;
-            image = metadata.image;
+            title = metadata.title || title;
+            description = metadata.description || description;
+            image = metadata.image || image;
             siteName = metadata.siteName;
           }
         }