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/export_test.go

@@ -0,0 +1,9 @@
+package api
+
+// DeliveryGoroutines and QueueLength are deliveryGoroutines and
+// queueLength, exported for the external test package, which fills the
+// queue.
+const (
+	DeliveryGoroutines = deliveryGoroutines
+	QueueLength        = queueLength
+)