Webhooks: post each new incoming message (closes #7)

Each message a contact sends in a direct chat is posted to each webhook
on that chat, as JSON holding `chat_id` and the message record of the
messages endpoint. The event handler only queues it: `api.Deliveries`
serves a queue of 100 with four goroutines, reads the chat's webhooks
there, and makes one POST per webhook, with a 10-second timeout, no
retry and no redirect followed. A full queue or a failed post is logged
by ids, never by URL or text. `bot.Run` stops it last, once the chat
client has exited. Tests cover the payload and its routing, a full
queue, failures, and a webhook that never answers; the README documents
the payload and what delivery promises.

Model: opus-5-5

docs/TODO.md

@@ -27,6 +27,7 @@ with no deprecation warning.
 
 # Completed Steps
 
+- 2026-09-29 Each new incoming message is posted to its chat's webhooks
 - 2026-09-29 `POST`, `GET` and `DELETE` on a chat's webhooks, under
   `/api/v1/chats/{id}/webhooks`, kept in `$DATA_DIR/webhooks.json`
 - 2026-09-29 `GET` and `POST /api/v1/chats/{id}/messages`: a chat's