feat(stellar): Implement payment links (shareable)

[zeemscript]

Description

Create a payment link system that generates shareable URLs for payment requests. Users can pre-fill payment details including amount, item description, and recipient, making it easy to request payments via any channel.

Acceptance Criteria

• Generate unique, shareable payment URLs
• Support pre-filled amount, item/service description, and recipient address
• Create payment link creation and retrieval endpoints
• Track payment link usage and completion status

Files to modify/create

• src/modules/stellar/payment-link.service.ts
• src/modules/stellar/payment-link.controller.ts
• src/modules/stellar/entities/payment-link.entity.ts
• src/modules/stellar/dto/payment-link.dto.ts