fix(newsletter): serialize ObjectId to string in API response

Root cause: MongoDB ObjectId objects were being sent to frontend as-is,
which JSON.stringify converts to '[object Object]' string in data attributes.

Fix: Convert _id to string on server-side before sending to client.

This is the actual fix - previous attempts were client-side workarounds.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/controllers/newsletter.controller.js

@@ -224,9 +224,15 @@ exports.listSubscriptions = async (req, res) => {
       ...(options.source && { source: options.source })
     });
 
+    // Convert ObjectId to string for JSON serialization
+    const serializedSubscriptions = subscriptions.map(sub => ({
+      ...sub,
+      _id: sub._id.toString()
+    }));
+
     res.json({
       success: true,
-      subscriptions,
+      subscriptions: serializedSubscriptions,
       pagination: {
         total,
         limit: options.limit,