fix(bounties): reserve claims before payout

[rissrice2105-agent]

Closes #47

Summary

• reserve a funded bounty with UPDATE ... RETURNING
• stop immediately when the conditional update returns no row
• remove the follow-up claimant read that allowed same-DID concurrent requests to continue
• add a focused regression that keeps the atomic reservation before CoinPay payout preparation

Why

Two requests from the same DID could both pass the initial funded-state read. Only one update succeeded, but both later reads saw that DID as the claimant and both could prepare and broadcast a payout.

Validation

• node --test test/*.test.mjs (4/4)
• tsc -p apps/web/tsconfig.json --noEmit
• git diff --check

The local Next 16.2.7 route-handler guide was reviewed before finalizing the route change.