feat(stellar): Add QR code payment generation

[zeemscript]

Description

Implement QR code generation for payment requests using SEP-7 URI format. This enables easy mobile payments by scanning QR codes that contain all necessary payment information.

Acceptance Criteria

• Generate QR codes containing SEP-7 payment URIs
• Support encoding amount, asset, destination, and memo
• Provide endpoint to generate QR codes as images (PNG/SVG)
• Include QR code in payment link responses

Files to modify/create

• src/modules/stellar/qr-payment.service.ts
• src/modules/stellar/payment.service.ts (update)
• Add QR code generation library dependency
• src/modules/stellar/dto/qr-payment.dto.ts

[1CryptoQueen]

I'd like to work on this QR code payment feature. I've worked with SEP-7 URI standards and QR generation in production environments before, so I understand the importance of making this reliable and developer-friendly.

Here's my approach:

1. Library Selection: I'll install a lightweight QR generation library that supports both PNG and SVG output. I'll keep it small to avoid bundle bloat.

2. SEP-7 URI Construction: I'll build a proper URI builder that formats the payment request exactly to the SEP-7 specification — web+stellar:pay?destination=...&amount=...&asset=... I'll include support for optional fields like memo, memo_type, and message.

3. QR Generation: The service will accept payment details and return a QR code image. I'll support both PNG for general use and SVG for crisp rendering at any size.

4. Endpoint Design: I'll create a GET endpoint that accepts query parameters and returns the QR image with proper content-type headers. For example: /payments/qr?destination=G...&amount=10&asset=USDC

5. Payment Service Integration: I'll update the existing payment service to include QR data in payment link responses. This way, users get both the link and the QR code in one call.

6. Error Handling: I'll validate all inputs (destination address, amount, asset) and return clear error messages. Invalid addresses or unsupported assets will be caught early.

7. Testing: I'll add unit tests to verify URI construction and QR generation. I'll also test the endpoint returns proper image responses.

I can have a complete PR ready within 24 hours.

Looking forward to contributing!