Notify question authors via Resend when an admin posts the first answer.

Require email at registration (editable on profile), add a Resend mailer with idempotent first-answer sends, and keep answer saves independent of delivery.

internal/mail/recording.go

@@ -0,0 +1,34 @@
+package mail
+
+import (
+	"context"
+	"sync"
+)
+
+// Recording is a test Notifier that records calls.
+type Recording struct {
+	mu   sync.Mutex
+	Msgs []QuestionAnswered
+}
+
+func (r *Recording) NotifyQuestionAnswered(_ context.Context, msg QuestionAnswered) error {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	r.Msgs = append(r.Msgs, msg)
+	return nil
+}
+
+func (r *Recording) Len() int {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	return len(r.Msgs)
+}
+
+// Snapshot returns a copy of recorded messages.
+func (r *Recording) Snapshot() []QuestionAnswered {
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	out := make([]QuestionAnswered, len(r.Msgs))
+	copy(out, r.Msgs)
+	return out
+}