feat(payments): monitor escrow contract events

[Marvellous805]

Summary

• Poll configured Soroban escrow contracts and persist event cursors and event records.
• Reconcile release/refund events with local PaymentIntent state and alert on unmatched terminal events.
• Expose an admin endpoint and document configuration and operations.

Closes #86

Verification

• git diff --check passed.
• Prisma generation, typecheck, and the focused unit test could not be run: npm dependency installation stalled during dependency resolution on the USB drive.

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