Ignore uploaded show backgrounds

Background images are uploaded through the admin pages on each server
and named to match that server's database, so they stay out of git.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.gitignore

@@ -31,3 +31,7 @@ Temporary Items
 
 # Database credentials — keep out of version control
 config/db_credentials.php
+
+# Show backgrounds are uploaded through the admin pages on each server
+images/backgrounds/*
+!images/backgrounds/.gitkeep