feat(contracts): add optional approver field and approve_invoice to i…

[aji70]

Summary

Adds an optional approval step to the invoice workflow. When an invoice is created with an approver address, approve_invoice() must be called by that approver before release() can execute.
Invoices without an approver are unaffected.

Changes

• types.rs — added approver: Option and approved: bool fields to the invoice struct; added approver: Option field to InvoiceOptions struct
• lib.rs — added approve_invoice() requiring approver auth and setting approved = true; updated release() to gate on approved when an approver is set; updated all invoice creation paths to
  pass approver parameter

Testing

• Test: invoice with approver blocks release until approved, then releases successfully
• Test: release with unapproved approver panics with "awaiting approval"
• Test: invoice without approver releases without any approval gate
• All existing cargo tests pass; cargo clippy passes with zero new warnings

Closes #25

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