Add Contract Tier

Add a tier to an existing contract (PRO-2569 / PRO-2675).

slots_purchased may be 0 — a 0-slot tier is a valid transfer-only upgrade/downgrade target that only ever fills via transfers from other tiers. A price is required either way (PRO-2768): this is the path for offering a catalog tier introduced after the contract was created. The tier's name + daily MCP token cap come from the referenced catalog entry; only slots + price are per-contract.