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

docs/TODO.md

@@ -30,6 +30,7 @@ with no deprecation warning.
 - 2026-09-29 Exact results past the range of a double, such as `2^1200`,
   are written to 17 significant digits instead of being refused, and a
   fractional power of such a number, such as `(2^1200)^0.5`, is answered
+- 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