Generalize post reply notifications (#6)

## Summary
- Notify the direct parent post author for replies throughout nested conversations
- Skip root creation, self-replies, edits, disabled mail, and recipients without email
- Link directly to each reply and use per-reply Resend idempotency

Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/mail/recording.go

@@ -8,10 +8,10 @@ import (
 // Recording is a test Notifier that records calls.
 type Recording struct {
 	mu   sync.Mutex
-	Msgs []QuestionAnswered
+	Msgs []PostReply
 }
 
-func (r *Recording) NotifyQuestionAnswered(_ context.Context, msg QuestionAnswered) error {
+func (r *Recording) NotifyPostReply(_ context.Context, msg PostReply) error {
 	r.mu.Lock()
 	defer r.mu.Unlock()
 	r.Msgs = append(r.Msgs, msg)
@@ -25,10 +25,10 @@ func (r *Recording) Len() int {
 }
 
 // Snapshot returns a copy of recorded messages.
-func (r *Recording) Snapshot() []QuestionAnswered {
+func (r *Recording) Snapshot() []PostReply {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	out := make([]QuestionAnswered, len(r.Msgs))
+	out := make([]PostReply, len(r.Msgs))
 	copy(out, r.Msgs)
 	return out
 }