feat: add tip support to pay() — optional gratuity on top of invoice amount

[MarcusDavidG]

Summary

Allow payers to include an optional tip amount on top of the invoice amount when calling pay().

Motivation

Freelancers and service providers often receive gratuities. Having tip support on-chain makes it transparent and eliminates the need for a separate transfer.

Proposed Change

• Add optional tip: Option<i128> param to pay()
• Transfer tip separately to a configurable tip recipient (creator by default)
• Emit a tip_received event with the tip amount
• Store tips in the Payment struct (field already exists as tip: i128)

Acceptance Criteria

• tip is transferred to creator on pay()
• tip amount stored in Payment struct
• tip_received event emitted
• test: tip is isolated from invoice funded amount
• test: invoice can still release independently of tip

Labels

enhancement, good first issue