feat: add invoice templates — reuse recurring configs without deploying new invoices

[MarcusDavidG]

Summary

Add a create_template() function that stores a reusable invoice configuration on-chain. Templates can be instantiated into live invoices with create_from_template(template_id).

Motivation

Teams and DAOs often send the same invoice structure repeatedly. Templates reduce transaction costs and UI friction.

Proposed Change

• Add InvoiceTemplate type with same fields as Invoice minus deadline and funded
• Add template_key(id) storage key
• Add create_template() and create_from_template(template_id, deadline)
• Template counter separate from invoice counter

Acceptance Criteria

• Template stored with unique ID
• Invoice created from template matches template fields
• test: create template and instantiate
• test: template not found panics with clear message