Update a sponsored ad pre-broadcast (sponsored_unpaid OR sponsored_paid)

Update a sponsored ad pre-broadcast.

Auth model (round-4 of 2026-05-08 post-mortem):

• Buyer (matched by buyer_wallet via JWT) can edit creative fields (headline, media, x_text, source) on sponsored_unpaid AND sponsored_paid.
• Editorial staff (is_staff) can edit creative fields plus tags and owner on the same statuses.
• Pricing (tier, coupon_code) is frozen at payment — even staff cannot change pricing on sponsored_paid.
• url is never editable.
• Material edits (media/headline/x_text/source) on a creative_pre_approved ad require require_reapproval=true to acknowledge the flag will be reset.

For post-broadcast (sponsored_approved) edits, use /api/v1/ad/<id>/edit-live/.