feat: implement append-only audit log (#50)

[obedebuka41-dotcom]

Summary

• Added AuditEntry struct with action, actor, and timestamp fields to types.rs
• Added Cancelled status to InvoiceStatus enum
• Implemented append_audit_entry() helper to log state changes
• Implemented get_audit_log() view function to retrieve entries
• Added audit logging to: pay(), release(), refund(), cancel_invoice(), extend_deadline()
• Added tests verifying log entries are correctly recorded

Testing

• test_audit_log - verifies pay and auto-release log entries (2 entries)
• test_audit_log_with_cancel - verifies cancel log entry
• test_audit_log_with_extend - verifies extend deadline log entry

Closes #50