Add payment send API

[lukaIvanic]

Summary

• add a POST /payments/send endpoint for creating a payment record
• add GET /payments/list so sent payment records can be inspected
• store payment records in the existing in-memory/zustand database with amount, recipient, currency, memo, status, and timestamp
• add a Bun route test for the send/list flow

Test

• npm_config_cache=/Users/lukaivanic/projects/organization/llm_money_making_experiment/folder_2/.npm-cache npx --yes bun test
• npm_config_cache=/Users/lukaivanic/projects/organization/llm_money_making_experiment/folder_2/.npm-cache npx --yes bun test tests/routes/payments/send.test.ts
• npm run build
• npx tsc --noEmit

/claim #1

Refs #1

[lukaIvanic]

Small review note: this PR is now fully revalidated after the default-currency fix. Current verification passed with npx --yes bun test, focused bun test tests/routes/payments/send.test.ts, npm run build, and npx tsc --noEmit. It includes /claim #1 in the PR body for the bounty.