fix(payment-requests): validate memo length against Stellar 28-byte limit (#49)

[ghzhost]

Summary

Closes #49

Enforces Stellar protocol's 28-byte maximum length constraint on payment_requests.memo both in application logic and in Postgres.

Changes

1. Application Validation (PaymentRequestService::validate_create_request):
   • Added validation enforcing that memo.len() <= 28 (checked in bytes, not character count).
   • Rejects oversized memos with AppError::Validation before any DB insert.
2. Database Constraint (migrations/012_add_payment_request_memo_check.sql):
   • Added CHECK (memo IS NULL OR octet_length(memo) <= 28) on payment_requests table for defense-in-depth.
3. Unit Tests (src/services/payment_request.rs):
   • Tested ASCII memos (28 bytes accepted, 29 bytes rejected).
   • Tested multi-byte UTF-8 boundary (e.g. 7 4-byte emojis = 28 bytes accepted; 8 4-byte emojis = 32 bytes rejected despite only 8 characters).