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

internal/api/api.go

@@ -1,6 +1,7 @@
 // Package api is the bot's HTTP API, through which another program
 // reads the bot's chats, sends messages in them and registers webhooks
-// on them.
+// on them. Deliveries posts to those webhooks each message a contact
+// sends.
 //
 // Every request must carry the credential, as "Authorization: Bearer
 // {credential}"; with no credential configured, every request is
@@ -9,6 +10,8 @@
 // Handlers call the chat client on the request's own goroutine. Doing
 // so from the chat client's event handler would wait forever, since
 // that goroutine also delivers the responses (see simplex.EventHandler).
+// For the same reason, the event handler hands messages to Deliveries
+// without waiting.
 package api
 
 import (