feat(backend): add invoice memo prefix validator (#272)

[s6pa1rta3n-lab]

Description

This PR implements hasInvoiceMemoPrefix(memo) to ensure memo validation enforces invoice-style prefixes (INV-).

Tasks Completed

• Implement hasInvoiceMemoPrefix(memo) in backend/src/utils/memo-prefix-check.ts
• Add unit tests with edge cases in backend/tests/memo-prefix-check.test.ts
• Add fixture data for tests in backend/tests/fixtures/memo-prefix-check.fixture.ts
• Import and use in backend/src/utils/memo.ts (minimal diff)

Acceptance Criteria Verification

1. Exclusive paths touched: Touches exactly the 4 requested files under backend/.
2. Tests passing: All 189 unit & contract tests pass (npm test).
3. Typecheck clean: TypeScript validation passes with 0 errors (npm run typecheck).
4. English only: All comments and strings in English.

Closes #272

Payout Routing

• EVM (Base/Arbitrum/Polygon/ETH): 0xF46C9F6d70C50BF81ef3588AB523a90a594a2F89
• Stellar: GCL6OXAMLD75BMTINA6EMRUDWK5THQUSHMYNLSNBCJAPZJHNYJTUNIBC