feat: add invoice deadline extension

[iamcracked-dev]

Closes #412

Changes

• Added an admin-only extend_deadline entrypoint for open funding invoices.
• Rejects non-open invoices and deadline values that do not move forward.
• Emits a deadline_extended event with the invoice id plus old and new ledger deadlines.
• Refreshes the funding invoice persistent TTL when records are read, written, or checked.

Testing

• Not run, per request.

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