Gasless USDC deposit for Base multisigs (x402)

[Huygon764]

Problem:

• Funding a Base multisig today requires the depositor to hold ETH on Base to pay gas and to run a standard ERC-20 transfer themselves. This blocks users who only hold USDC and forces every AI agent or external script to integrate a custom PolyPay flow. There is no standardized, gasless, non-custodial way to fund a multisig, and no public endpoint that external x402 clients can call.

Expectation:

• Human users: from the Portfolio drawer of any Base multisig, click Deposit,
  enter 1–10,000 USDC, sign once, done. No ETH required.
• Agents / scripts: GET/POST /api/x402/deposit/{multisigAddress} is x402 v1
  compliant; any x402 client (x402-fetch, Python x402, etc.) can fund a registered
  Base mainnet (8453) or Base Sepolia (84532) multisig.
• Non-custodial: EIP-3009 signature locks both recipient and amount; a
  facilitator pays gas and cannot redirect funds. Each authorization has a unique
  nonce + 5-minute validity; duplicates are rejected.
• Rate limits enforced per IP and per multisig to prevent abuse.