Sprint 3: integrate on-chain payouts

[sacha-l]

Features:

• Each event has funds in escrow contract
• Do payouts from the app
• Contract handles curators and project leads
• Contract handles multisig
• Attach proof of milestone completion
• Ability for judges to vote using contract

[sacha-l]

Deferring out of Phase 1 per the Stadium revamp spec (docs/stadium-revamp-phase-1-spec.md §8.1) and docs/revamp-context.md §"What Phase 1 is explicitly not doing".

Context doc: "We are not touching the payout / multisig plumbing. M2 payments work; don't touch them. Dogfooding does not have payouts."

Leaving this open as Phase 3+ work. Revisit post-revamp once the program abstraction (programs table, program_applications) is in place — that's the layer this on-chain work would plug into.

[sacha-l]

Launch decision: manual settlement. Payouts for launch are manual — an admin sends funds out-of-band and records the payment with a transaction-proof URL (confirmPayment records; it does not execute). Documented in docs/payouts.md via PR #90.

This issue's automated on-chain execution intent now overlaps with #85 ("Automated multi-chain payout execution — EVM / Solana"), which is broader (covers Substrate + EVM + Solana). Recommend consolidating: close this in favour of #85 so there is one tracking issue for automated payouts. Flagging here rather than closing unilaterally — close it if you agree.