feat(watcher): headless payment confirmation over public RPC + signed webhooks

The watcher polls balanceOf on per-invoice receive addresses via plain
JSON-RPC — no proving, no discovery service — marks invoices paid,
best-effort resolves the tx hash from Transfer events, and delivers
HMAC-signed payment.confirmed webhooks with retry. Zero dependencies.

Verified: 12 core tests + 8 watcher tests green, and e2e-live.mjs
confirms against Starknet MAINNET (balance check → paid → webhook with
valid signature) without spending anything.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>