[node-sdk] Webhook receiver middleware (Express, Hono, Next.js)

[KayleeWilliams]

Why

Pairs with verifyWebhook(). Users want dsar.webhooks.middleware() for their framework - not "here is how to do HMAC yourself." Middleware is the ergonomic shape that makes the SDK feel like a batteries-included product.

Scope

• @dsar/node-sdk/webhooks/express
• @dsar/node-sdk/webhooks/hono
• @dsar/node-sdk/webhooks/next (App Router route.ts helper)
• Each parses the request, verifies the signature, deserializes into a typed event, and invokes a user-supplied handler per event type
• Event-type dispatcher: on('deletion.requested', handler), on('export.requested', handler), etc.
• Typed handler signatures so TS infers payload shape per event
• Tests for each framework adapter

Priority: P1 - beta blocker
Depends on: verifyWebhook() helper