fix(backend): auto-create event ChatRoom on first check-in

Problem:
- User got "Chat room not found" error when trying to send messages
- Event ChatRooms were only created by seed script, not for manually
  created events
- Event "Another Dance Event" (ID: 420) was missing its ChatRoom

Root Cause:
- Seed script (seed.js:179-188) correctly creates ChatRooms for events
- But events created outside of seed (CLI, manual DB insert) didn't
  create ChatRooms
- Socket handler requires ChatRoom to exist before accepting messages

Solution:
1. Added defensive check in check-in handler (POST /api/events/checkin/:token)
2. Automatically creates ChatRoom if missing when first user checks in
3. Logs creation for debugging: "Created missing chat room for event: {slug}"

Impact:
- Existing events without ChatRooms will get them on next check-in
- Future manually-created events will work correctly
- No breaking changes - all 223 tests pass

Changes:
- backend/src/routes/events.js: Added ChatRoom existence check and
  auto-creation logic (lines 256-272)

Note: Manually created ChatRoom for event ID 420 to fix immediate issue

backend/src/routes/events.js

@@ -253,6 +253,24 @@ router.post('/checkin/:token', authenticate, async (req, res, next) => {
       });
     }
 
+    // Ensure event chat room exists (create if missing)
+    const existingChatRoom = await prisma.chatRoom.findFirst({
+      where: {
+        eventId: event.id,
+        type: 'event',
+      },
+    });
+
+    if (!existingChatRoom) {
+      await prisma.chatRoom.create({
+        data: {
+          eventId: event.id,
+          type: 'event',
+        },
+      });
+      console.log(`✅ Created missing chat room for event: ${event.slug}`);
+    }
+
     // Add user to event participants
     const participant = await prisma.eventParticipant.create({
       data: {