Circle Gateway: accept cross-chain USDC from buyers

[manoahLinks]

Summary

Let buyers pay for tickets in USDC from any Circle Gateway-supported chain (Ethereum, Arbitrum, Optimism, Polygon, Base, Avalanche, Solana) and settle the deposit on Base for ticket minting. Chain-agnostic UX without manual bridging.

Scope

• Add a "pay with crypto (any chain)" option alongside fiat (Paystack/Monnify) on checkout.
• On buyer intent, return a Gateway deposit instruction (address + amount on the chain they selected).
• Listen for `gateway-deposit.success` webhook (Webhooks: Circle webhook handler (/webhooks/circle) #65) → trigger ticket mint on Base.
• Fallback: if Gateway fails or is slow, fall back to standard single-chain USDC deposit to buyer's Circle wallet on Base.
• Document chain selection UX + fee disclosure (Gateway charges ~$0.01 routing fee).

Implementation notes

Invoke `circle:use-gateway` for deposit/mint/burn intent API. Never send USDC directly to the Gateway contract — API only.

Acceptance criteria

• Buyer on Arbitrum can buy a ticket; deposit confirmed on Base < 60s
• Webhook flow unified with other crypto deposits
• Frontend shows per-chain fees pre-purchase
• Fallback tested: Gateway timeout → direct USDC deposit works

Depends on

• Circle: SDK setup + entity secret registration + wallet set bootstrap #61, DB: Prisma migration — add Circle wallet fields to users + blockchain_transactions #62, Circle: Treasury wallet setup (platform signing wallet) #63, Webhooks: Circle webhook handler (/webhooks/circle) #65, Smart contracts: Deploy Diamond to Base Sepolia + Base mainnet #66