HTTP API: 404 for a chat id the list of chats leaves out (closes #5)

The chat client keeps contact records that GET /api/v1/chats does not
list, such as the bot's own profile (1 on a new profile) and a contact
it creates itself (2), and reads or sends in them when asked. The
message endpoints now look the id up, through chatID in
internal/api/chats.go, in the same list the chats endpoint answers
with, and answer 404 for any id not in it. The webhook endpoints can
call chatID too.

Model: opus-5-5

README.md

@@ -180,7 +180,7 @@ A message has:
   it.
 
 A `count` other than a whole number from 1 to 100 gets `400`, and an
-`id` that is not one of the bot's chats gets `404`.
+`id` that `GET /api/v1/chats` does not list gets `404`.
 
 ### `POST /api/v1/chats/{id}/messages`
 
@@ -208,7 +208,7 @@ curl -H "Authorization: Bearer $TOKEN" -H 'Content-Type: application/json' \
 Nothing is sent, and the answer is:
 
 - `400` if the body is not JSON of that shape, or `text` is empty;
-- `404` if `id` is not one of the bot's chats;
+- `404` if `GET /api/v1/chats` does not list `id`;
 - `409` if the contact cannot receive messages: they have deleted their
   chat with the bot (`contact_deleted` is `true`), or have not finished
   connecting;