feat(gateway): multi-merchant support — sub-accounts under a single publishable key

[fabicabvq06]

Summary

Adds multi-merchant support to the Connect Gateway: a single platform pk_/sk_ pair can host many isolated sub-merchants, each with isolated escrow settlement and webhook delivery, plus an admin listing of sub-merchants and their settled volume.

All new columns are nullable and all new tables additive, so existing single-merchant integrations are unchanged.

What's included

• Data model — new Merchant (sub-account owned by a platform ApiKey) and MerchantSettlement (idempotent per-escrow ledger) models, plus a nullable merchantId on CheckoutSession, WebhookEndpoint, WebhookEvent, and Subscription, with a single additive migration.
• Sessions — POST /v1/session accepts an optional merchantId, validated to be an active sub-merchant owned by the authenticated key (unknown/disabled/foreign → 400).
• Escrow settlement — escrows inherit the session's merchantId; on release the amount/asset are recorded against that sub-merchant's settled volume (idempotent on escrowId, fire-once across both release paths).
• Webhook isolation — dispatch is strictly scoped: an event with merchantId = M reaches only endpoints scoped to M; a platform event reaches only platform (null) endpoints. Endpoints can be registered with a merchantId; inbound webhooks and subscription events carry/inherit it.
• Admin API — POST/GET /admin/keys/:keyId/merchants (create / list with per-asset settled volume) and POST /admin/merchants/:id/disable.
• Docs — new MULTI_MERCHANT.md and merchantId additions to WEBHOOKS.md.

Acceptance criteria

• ✅ Creating a session with a merchantId routes settlement to that sub-merchant only.
• ✅ Webhooks for one sub-merchant's escrow never reach another sub-merchant's endpoint (nor a platform endpoint, and vice-versa).
• ✅ A platform can list its sub-merchants and per-sub-merchant volume via the gateway API.

Testing

• Gateway test suite: 178/178 passing (TDD per change).
• tsc --noEmit clean, Biome clean.

Closes #44

[aguilar1x]

lgtm!