Implement LP whitelist for private invoice markets

[Nursca]

Description
Some businesses may want to offer invoices exclusively to trusted LPs (e.g. their own treasury, a specific VC). An optional LP whitelist on invoices enables private invoice markets within the public protocol.

Requirements and context

• submit_invoice() accepts optional allowed_lps: Option<Vec<Address>>
• If set, fund_invoice() rejects callers not in the whitelist with Error::LPNotWhitelisted
• Store whitelist on invoice struct (cap at 10 addresses to limit storage)
• Emit whitelist in InvoiceSubmitted event
• Write tests: public invoice (no whitelist), private invoice (whitelist), non-whitelisted LP rejected

Suggested execution

git checkout -b feat/lp-whitelist

• Add optional whitelist to invoice struct
• Add check in fund_invoice()
• Write tests

Example commit message
feat: add optional LP whitelist for private invoice markets