Handle Inbound Sms

Handle inbound SMS/MMS messages from Twilio.

Validates the signature and the field shapes, then stores the payload and queues it. Everything else — org resolution, spam, media, the agent — runs in the inbound worker against the stored row.

Failures are NOT swallowed: if the payload cannot be stored, Twilio gets a 500 rather than an empty TwiML that pretends the message was handled.