HTTP API: server, credential and the list of chats (closes #4)

The bot now serves an HTTP API on `PORT` (default 8080) beside the chat client, whose WebSocket stays on 127.0.0.1 inside the container. Every request needs `Authorization: Bearer` with the credential from the file named by `API_TOKEN_FILE`, compared in constant time; with no credential configured every request is refused, `OPTIONS *` included. `GET /api/v1/chats` lists the bot's chats. Responses carry the security headers from the repository policies; bodies, requests and the server are time- and size-bounded. The chat client stops only after the API has finished its requests.

Disclosures: `contact_deleted` is an extra field; 404 and 405 answer in JSON; requests net/http cannot parse are refused by net/http without the security headers; three gosec findings are suppressed as false positives.

Model: opus-5-5

internal/simplex/client.go

@@ -168,6 +168,18 @@ func (c *Client) SetAddressSettings(
 	return c.command(ctx, cmd, TypeUserContactLinkUpdated, nil)
 }
 
+// Contacts returns the user's contacts: everyone it has a direct chat
+// with.
+func (c *Client) Contacts(ctx context.Context, userID int64) ([]Contact, error) {
+	var r struct {
+		Contacts []Contact `json:"contacts"`
+	}
+
+	err := c.command(ctx, cmdListContacts(userID), TypeContactsList, &r)
+
+	return r.Contacts, err
+}
+
 // SendText sends a text message to a contact, as a reply to the message
 // quotedItemID (0 for none). It does not wait for the chat client to
 // accept it; a failure is logged when the client's answer arrives.