nwc: fix make_invoice timestamps (ms -> seconds) breaking NWC clients

generate() stores `created` in milliseconds (Date.now()), but the NWC
make_invoice response returned it directly as created_at and computed
expires_at = created_at + expiry. NWC timestamps are Unix SECONDS (NIP-47),
so clients parsed a ~1.78e12 expires_at that overflowed their date types —
e.g. ptcpay's .NET DateTimeOffset, whose max is 253402300799 (issue #84:
"creating lightning invoices via NWC fails ... Valid values are between
-62135596800 and 253402300799"). Convert created to seconds before use.

The other NWC handlers were already correct (lookup_invoice reads seconds
straight from cln; list_transactions divides p.created by 1000). Only
make_invoice leaked milliseconds.

Fixes #84

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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