Fix date format issues in events API

- Convert Unix timestamps to ISO strings before sending to frontend
- Store dates as ISO strings in database during event creation
- Fix created_at, expires_at, and archived_at date conversions

This resolves the "Invalid time value" error that occurred when viewing
the events page due to SQLite returning dates as Unix timestamps.

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

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

backend/src/routes/adminEvents.js

@@ -77,8 +77,8 @@ router.post('/', adminAuth, [
       welcome_message,
       color_theme,
       share_link: shareLink,
-      expires_at,
-      created_at: new Date()
+      expires_at: expires_at.toISOString(),
+      created_at: new Date().toISOString()
     });
     
     // Log activity
@@ -180,10 +180,14 @@ router.get('/', adminAuth, async (req, res) => {
       return acc;
     }, {});
 
-    // Add photo counts to events
+    // Add photo counts to events and convert dates
     const eventsWithCounts = events.map(event => ({
       ...event,
-      photo_count: photoCountMap[event.id] || 0
+      photo_count: photoCountMap[event.id] || 0,
+      // Convert Unix timestamps to ISO strings
+      created_at: event.created_at ? new Date(event.created_at).toISOString() : null,
+      expires_at: event.expires_at ? new Date(event.expires_at).toISOString() : null,
+      archived_at: event.archived_at ? new Date(event.archived_at).toISOString() : null
     }));
 
     res.json({