Add Blooio as an optional iMessage channel

[davext]

Summary

• Adds Blooio's v4 API (https://api.blooio.com/v4) as an optional iMessage provider next to Linq. Existing Linq deployments are unchanged: sign-in still uses Linq when LINQ_CONNECTOR is set.
• Inbound messages are accepted only from a verified phone account, after X-Blooio-Signature is checked. Replies, native Tapbacks, rich links, read receipts, and scheduled reports go back to the same Blooio chat.
• When Linq is not configured, phone sign-in codes are sent through Blooio. Set BLOOIO_API_KEY, BLOOIO_WEBHOOK_SECRET, and optionally BLOOIO_FROM_NUMBER, and point a webhook at /webhooks/blooio.

Test plan

• With only BLOOIO_API_KEY and BLOOIO_WEBHOOK_SECRET set, request a sign-in code and confirm it arrives by iMessage.
• Sign in, text the Blooio number, and confirm an unverified number is ignored while the verified number starts a conversation.
• Confirm send_message, a Tapback, and a rich link are delivered, and that a bad signature is rejected.
• Create a reminder in that thread and confirm the scheduled result is delivered back to the same chat.
• With LINQ_CONNECTOR set and Blooio unset, confirm Linq sign-in and delivery still behave as before.