Add Stellar USDC trustline validation before payouts

[Sikkra]

Summary

• Move Stellar recipient validation into src/lib/stellar.ts so public-key format, on-chain account existence, and configured USDC trustline checks are shared.
• Reuse the shared validator in cycle payouts and cancellation refunds so both direct USDC transfer paths fail before sending when the recipient cannot receive USDC.
• Add focused coverage for the trustline helper, payout validation reuse, and cancellation refund preflight ordering.

Closes #37

Verification

pm test -- --runTestsByPath src/lib/tests/stellar.test.ts src/server/services/tests/payout.service.test.ts --runInBand

pm test -- --runTestsByPath src/server/services/tests/circle.service.test.ts --runInBand -t cancelCircle

Notes

pm run type-check is currently blocked by existing repository-wide TypeScript issues outside this patch, including migration typing errors, missing Jest globals/types, missing exported app types, and existing API/service type errors.