Whatsapp Webhook

Parse the WhatsApp multipart, build a channel-blind InboundMessage, and dispatch to the generic core.

Multipart contract (#810, #814): payload is the JSON-encoded WhatsAppWebhookRequest body; audio, image, and document are mutually-exclusive optional file fields. At least one of text/audio/image/document must be present.

Returns 200 after the durable turn is enqueued; the coach run or invite send happens in the channel worker.