feat: add max_payment_amount guard to create_invoice — cap individual payment size

[MarcusDavidG]

Summary

Add an optional max_payment_amount: Option<i128> to InvoiceOptions that caps the maximum a single payer can contribute in one pay() call.

Motivation

Prevents whale dominance in partially-funded invoices and enables fair contribution mechanics for DAO treasury payments.

Proposed Change

• Add max_payment_amount: Option<i128> to InvoiceOptions
• Enforce cap in pay() with assert
• Store in Invoice struct
• Emit in invoice_created event

Acceptance Criteria

• max_payment_amount stored in Invoice
• pay() rejects amounts above cap
• test: payment at cap succeeds
• test: payment above cap panics
• test: None cap allows any amount