Set notify_external_testers explicitly for TestFlight assignment

fastlane warned it was relying on App Store Connect's default (true) for
notifying external testers. Set notify_external_testers:true explicitly so
behavior doesn't depend on an Apple-side default that could change.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/build-submit-ios.yml

@@ -241,7 +241,8 @@ jobs:
             app_identifier:"xyz.blueskyweb.app" \
             app_version:"$APP_VERSION" \
             build_number:"$BUILD_NUMBER" \
-            groups:"QA Team"
+            groups:"QA Team" \
+            notify_external_testers:true
 
       - name: 🔔 Notify Slack of Production Build
         if: ${{ inputs.profile == 'production' }}