HTTP API: a chat's recent messages, and sending a message (closes #5)

`GET /api/v1/chats/{id}/messages?count=N` returns a chat's recent messages, oldest first (`count` 1 to 100, default 20), and `POST` to the same path sends a text message and returns it once the chat client has taken it. Both answer `404` for any id the chats list does not show, looked up in the same contact list. The message record is defined once, for the webhooks to reuse.

Disclosures: items are read five at a time, because one item can be many times its text and 100 at once could pass the 16 MiB read limit; text too long gets `413`, a contact who deleted the chat `409` (unverified for one still connecting); a query the server cannot read gets its own `400`.

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"