docs: research WhatsApp catering bot APIs and propose architecture

Research spike for OAK-8. Compares Meta WhatsApp Cloud API against Twilio
and other BSPs, proposes infrastructure and message flow, and recommends a
provider. No bot implementation.

Key findings:
- Meta's Groups API exists but caps groups at 8 participants, is invite-only,
  and requires Official Business Account status (press-notability gated), so
  the existing group-chat workflow cannot be automated. Group broadcast stays
  manual, with a wa.me deep link preserving reply attribution.
- Recommends developing against Twilio's sandbox (no Meta account or business
  verification required) and shipping production on Meta Cloud API direct,
  behind a thin provider adapter.
- Flags that service messages and in-window utility templates become billable
  on 2026-10-01.

Author: stephenhungg