Fixes from June to October

A batch of correctness fixes across spaces, Sell, worksheets, attachments, and invites:

• Duplicated spaces keep their setup: each app instance is copied with its own name and settings, and module prerequisite locks and ordering carry over.
• One-time offer coupons: the coupon is applied before checkout, so the platform fee is calculated on the discounted price.
• Coupon redemption caps: the cap sits on the promotion code, so a forever discount keeps applying on later invoices after the code hits its redemption limit.
• Checkout errors are visible: a failed checkout shows an error instead of leaving the Join button doing nothing.
• Long worksheet answers are readable in the response overview.
• PDF attachments: downloads no longer send PDFs through the image optimizer, so a PDF opens instead of failing like a missing file.
• Invites: blank email rows are ignored, and a validation error is shown when an address is still invalid.