Set additional email-user credits

Sets the account's additional email-user credits (the per-user addon) to a target total, mirroring the Settings > Subscription dashboard flow. Adds, updates, or removes the addon line item on the main plan subscription (the change is prorated and charged immediately to the default payment method), then updates email_user_limit_additional_credits and email_user_limit.

quantity is the target TOTAL additional credits (not a delta), so the call is idempotent and safe to retry. Effective email user limit = base plan limit

• additional credits.

Floor: additional credits can never be set below max(0, email_users_used - email_user_limit_base) — i.e. the total allowance can never drop below the mailboxes currently in use. To go lower, delete email users first.

Not available for reseller / Connected-Account sub-accounts (returns 400 — use the dashboard).