fix: dashboard activity endpoint JSON parsing error

- Add safe JSON parsing for activity metadata in dashboard endpoint
- Handle cases where PostgreSQL returns JSON columns as objects
- Prevent 500 errors when metadata contains invalid JSON

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

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

backend/src/routes/adminDashboard.js

@@ -122,7 +122,16 @@ router.get('/activity', adminAuth, async (req, res) => {
       actorType: activity.actor_type,
       actorName: activity.actor_name,
       eventName: activity.event_name,
-      metadata: activity.metadata ? JSON.parse(activity.metadata) : {},
+      metadata: (() => {
+        try {
+          if (!activity.metadata) return {};
+          if (typeof activity.metadata === 'object') return activity.metadata;
+          return JSON.parse(activity.metadata);
+        } catch (e) {
+          console.warn('Failed to parse metadata for activity:', activity.id, e.message);
+          return {};
+        }
+      })(),
       createdAt: activity.created_at
     }));