Automated multi-chain payout execution (EVM / Solana)

[sacha-l]

Context

PR #84 (multi-chain sign-in) lets a project register a payout address on Substrate, Ethereum, or Solana (projects.donation_chain). However, automated payout execution is still Substrate-only:

• server/api/services/payment.service.js constructs Asset Hub USDC transfers through a Polkadot multisig. It cannot send funds to an Ethereum or Solana address.
• confirmPayment only records a payment (appends to totalPaid with a proof URL), so recording works for any chain — but there is no automated send for EVM/Solana.

Result today: a project with donation_chain = 'ethereum' | 'solana' must be settled manually off-platform, then recorded.

Scope

Design and build automated payout execution for non-Substrate payout chains:

• An EVM transfer path (funded treasury wallet or Safe; ERC-20/native send; gas handling).
• A Solana transfer path (SPL token transfer; fee payer).
• Decide the treasury model per chain (single wallet vs. multisig/Safe) and where keys live.
• Surface chain-appropriate UI in ConfirmPaymentModal and guard the Substrate multisig path from being fed a non-Substrate recipient.

Test scenarios

• On the admin payment flow, confirming a payout for a project with donation_chain = 'ethereum' → an on-chain EVM transfer is initiated (or a clear, auditable manual-settlement record is created).
• On the admin payment flow, a Substrate-chain project → the existing Asset Hub multisig path runs unchanged.
• On payment.service.js, a non-Substrate recipient passed to the multisig path → rejected with a clear error rather than building a broken extrinsic.

Notes

Out of scope for PR #84 by design — flagged there. Larger effort: needs treasury wallets/keys and a security review.