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

internal/api/chats_test.go

@@ -54,7 +54,7 @@ func TestNoChats(t *testing.T) {
 func TestChatsFailure(t *testing.T) {
 	t.Parallel()
 
-	rec := request(t, newAPI(credential, &fakeClient{err: errChat}),
+	rec := request(t, newAPI(credential, &fakeClient{contactsErr: errChat}),
 		http.MethodGet, chatsPath, bearer)
 
 	want := `{"error":"the chats could not be read"}` + "\n"