Define adapter interfaces and fake implementations

internal/adapters/notify/sender.go

@@ -3,13 +3,20 @@ package notify
 
 import "context"
 
-// Sender is a placeholder adapter interface for notifications.
+// Sender is the adapter boundary for notifications.
 type Sender interface {
-	Send(ctx context.Context, msg Message) error
+	Send(ctx context.Context, req SendRequest) (SendResult, error)
 }
 
-// Message is a placeholder notification payload.
-type Message struct {
-	Subject string
-	Body    string
+// SendRequest describes one notification request.
+type SendRequest struct {
+	Subject  string
+	Body     string
+	Metadata map[string]string
+}
+
+// SendResult describes notification send outcome metadata.
+type SendResult struct {
+	ProviderMessageID string
+	Metadata          map[string]any
 }