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/api.go

@@ -1,5 +1,5 @@
 // Package api is the bot's HTTP API, through which another program
-// reads the bot's chats.
+// reads the bot's chats and sends messages in them.
 //
 // Every request must carry the credential, as "Authorization: Bearer
 // {credential}"; with no credential configured, every request is
@@ -47,6 +47,17 @@ const (
 type ChatClient interface {
 	// Contacts returns the contacts of the user userID.
 	Contacts(ctx context.Context, userID int64) ([]simplex.Contact, error)
+
+	// ChatItems returns the last count items of the chat with a
+	// contact, oldest first.
+	ChatItems(
+		ctx context.Context, contactID int64, count int,
+	) ([]simplex.ChatItem, error)
+
+	// SendMessage sends a text message to a contact and returns it.
+	SendMessage(
+		ctx context.Context, contactID int64, text string,
+	) (simplex.ChatItem, error)
 }
 
 // Params configures New.
@@ -86,6 +97,8 @@ func New(p Params) *http.Server {
 	})
 	router.Route("/api/v1", func(r chi.Router) {
 		r.Get("/chats", h.handleChats())
+		r.Get("/chats/{id}/messages", h.handleMessages())
+		r.Post("/chats/{id}/messages", h.handleSend())
 	})
 
 	return &http.Server{