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/web/server.go

@@ -510,6 +510,7 @@ func (s *Server) handleAnswer(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, "could not save answer", http.StatusInternalServerError)
 		return
 	}
+	s.notifyPostReply(root, root, reply, u)
 	location := "/questions/" + url.PathEscape(id) + "#post-" + url.PathEscape(reply.ID)
 	if isHTMX(r) {
 		w.Header().Set("HX-Redirect", location)