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

Each message a contact sends in a direct chat is posted to each webhook registered on that chat: `POST`, `Content-Type: application/json`, body `{"chat_id":N,"message":{...}}` with the same record the messages endpoint returns. The event handler only queues the message; four goroutines post it, one attempt each with a 10-second timeout and no retries, and with 100 already waiting a message is dropped and logged, so a slow webhook never delays the bot's replies. Posting stops last, after the chat client.

Disclosures: redirects are not followed; a logged failure can name the webhook's host but never its path, query or credentials; posts still in flight at shutdown are abandoned and logged.

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
+)