Implement invoice NFT minting on submission

[Nursca]

Description
Representing invoices as NFTs (Stellar NFT standard) makes them composable with the broader Stellar DeFi ecosystem. LPs could trade invoice claims on NFT marketplaces, dramatically improving liquidity.

Requirements and context

• On submit_invoice(), mint an NFT to the submitter representing their invoice
• NFT metadata: invoice ID, amount, due date, discount rate, token
• On fund_invoice(), transfer NFT to the LP as a claim receipt
• On mark_paid(), burn the NFT
• Follow the Stellar NFT standard
• Write full lifecycle tests including NFT transfers

Suggested execution

git checkout -b feat/invoice-nft

• Research Stellar NFT standard on Soroban
• Implement minting, transfer, and burn
• Write lifecycle tests

Example commit message
feat: represent invoices as NFTs on Stellar for DeFi composability