[neeerp]

Row 205, NX source 2: coupon codes. The Cash Shop already redeemed coupons (0xE6, b072), but the only way to make one was SQL.

• New !coupon <nx=N|mp=N|np=N|meso=N|item id[xN]>... [days=N] [code=CODE] (tier 2, GM). It takes one or more rewards, including cash items, which nothing else in the game gives.
• With no code=, the code is 16 random letters and digits, leaving out 0/O/1/I. With no days= (1–3650), the coupon never expires.
• Unknown items, amounts of 0 or less, and unknown keys get the usage line. A code that already exists gets "That code is taken."
• WorldAction::MakeCoupon is resolved in runtime/src/actor/client/gm.rs by db::coupon::create. The reply names the code, and the result is logged under gm_audit.

Tests:

• Unit test coupons_take_rewards_an_expiry_and_a_code.
• Harness test coupons_a_gm_makes_are_redeemed: a named code, a duplicate code that is refused, and a random 3-day code. Both redeem with 0x59, and the balances come back as (700, 0, 0) and then (700, 5, 0).
• gm_commands and cash_coupons pass.