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_test.go

@@ -19,6 +19,7 @@ import (
 
 	"plumber"
 	"plumber/internal/blob"
+	"plumber/internal/mail"
 	"plumber/internal/pacific"
 	"plumber/internal/store"
 )
@@ -483,7 +484,8 @@ func TestProfileAdminAnsweredListAndAvatarUpload(t *testing.T) {
 }
 
 func TestMutationsVoteAnswerHideAndCSRF(t *testing.T) {
-	srv, mem := newTestServer(t, Config{})
+	recording := &mail.Recording{}
+	srv, mem := newTestServer(t, Config{Mail: recording})
 	h := srv.Handler()
 	adminName := uniq("admin")
 	userName := uniq("user")
@@ -611,6 +613,13 @@ func TestMutationsVoteAnswerHideAndCSRF(t *testing.T) {
 	if got := rec.Header().Get("HX-Redirect"); got != "/questions/"+q.ID+"#post-"+adminReply.ID {
 		t.Fatalf("admin answer redirect = %q", got)
 	}
+	msgs := waitForMail(t, recording, 1)
+	if msg := msgs[0]; msg.ToEmail != user.Email ||
+		msg.RootID != q.ID ||
+		msg.ReplyID != adminReply.ID ||
+		msg.ReplyBody != adminReply.Body {
+		t.Fatalf("compatibility reply notification = %+v", msg)
+	}
 
 	rec = httptest.NewRecorder()
 	req = httptest.NewRequest(http.MethodGet, "/questions/"+q.ID, nil)