fix: prevent backend crash on archive when admin_email is null (#318)

Archiving an event with no admin_email queued an email_queue row with
recipient_email=null, violating the NOT NULL constraint. The error was
thrown inside the output.on('close') callback (detached from the caller),
becoming an unhandled rejection that crashed Node and dropped admin
sessions on bulk archive.

- Skip queueEmail when event.admin_email is null/empty (admin_email has
  been nullable since migration 073).
- Wrap the close handler in try/catch so any post-archive failure logs
  instead of crashing the process.

backend/src/services/archiveService.js

@@ -61,6 +61,7 @@ async function archiveEvent(event) {
     }
     
     output.on('close', async () => {
+      try {
         logger.info(`Archive created: ${archiveName} (${archive.pointer()} bytes)`);
 
         // Update database
@@ -82,11 +83,21 @@ async function archiveEvent(event) {
           }
         }
 
-      // Queue completion email
+        // Queue completion email — admin_email is nullable on events (migration 073);
+        // skip queueing rather than violating email_queue.recipient_email NOT NULL.
+        if (event.admin_email) {
           await queueEmail(event.id, event.admin_email, 'archive_complete', {
             event_name: event.event_name,
             archive_size: (archive.pointer() / 1024 / 1024).toFixed(2) + ' MB'
           });
+        } else {
+          logger.info(`Skipping archive_complete email for event ${event.slug}: no admin_email set`);
+        }
+      } catch (err) {
+        // Never let the close handler reject — it runs detached from the caller,
+        // and an unhandled rejection here crashes the backend process.
+        logger.error(`Post-archive cleanup failed for event ${event.slug}:`, err);
+      }
     });
     
     archive.pipe(output);