fix: sync rich-only private messages

internal/store/memory/message_send.go

@@ -45,6 +45,9 @@ func (s *MessageStore) Create(_ context.Context, msg domain.Message) (domain.Mes
 }
 
 func (s *MessageStore) SendPrivateText(_ context.Context, req domain.SendPrivateTextRequest) (domain.SendPrivateTextResult, error) {
+	if !req.HasContent() {
+		return domain.SendPrivateTextResult{}, domain.ErrMessageEmpty
+	}
 	fingerprint, err := store.PrivateSendFingerprint(req)
 	if err != nil {
 		return domain.SendPrivateTextResult{}, err

internal/store/memory/message_test.go

@@ -90,6 +90,35 @@ func TestMessageStoreSendPrivateTextCreatesBothOwnerBoxes(t *testing.T) {
 	}
 }
 
+func TestMessageStoreSendPrivateTextContentInvariant(t *testing.T) {
+	ctx := context.Background()
+	messages := NewMessageStore()
+
+	richOnly, err := messages.SendPrivateText(ctx, domain.SendPrivateTextRequest{
+		SenderUserID:    1000000001,
+		RecipientUserID: 1000000002,
+		RandomID:        191,
+		Date:            1700000191,
+		RichMessage:     &domain.MessageRichMessage{Blocks: validRichMessageBlocks},
+	})
+	if err != nil {
+		t.Fatalf("SendPrivateText rich-only: %v", err)
+	}
+	if richOnly.SenderMessage.Body != "" || richOnly.SenderMessage.RichMessage.IsZero() {
+		t.Fatalf("rich-only sender message = %+v, want empty body with rich payload", richOnly.SenderMessage)
+	}
+
+	_, err = messages.SendPrivateText(ctx, domain.SendPrivateTextRequest{
+		SenderUserID:    1000000001,
+		RecipientUserID: 1000000002,
+		RandomID:        192,
+		Date:            1700000192,
+	})
+	if !errors.Is(err, domain.ErrMessageEmpty) {
+		t.Fatalf("SendPrivateText empty err = %v, want ErrMessageEmpty", err)
+	}
+}
+
 func TestMessageStoreEditRichOnlyMessageUsesFinalContentState(t *testing.T) {
 	ctx := context.Background()
 	messages := NewMessageStore()

internal/store/postgres/message_rich_message_integration_test.go

@@ -3,6 +3,7 @@ package postgres
 import (
 	"bytes"
 	"context"
+	"errors"
 	"testing"
 	"time"
 
@@ -54,7 +55,6 @@ func TestSendPrivateRichMessageSurvivesReadPaths(t *testing.T) {
 		SenderUserID:    sender.ID,
 		RecipientUserID: recipient.ID,
 		RandomID:        time.Now().UnixNano(),
-		Message:         "rich",
 		RichMessage:     rich,
 		Date:            int(time.Now().Unix()),
 	})
@@ -121,4 +121,16 @@ func TestSendPrivateRichMessageSurvivesReadPaths(t *testing.T) {
 	if !sawEvent {
 		t.Fatal("no new_message event for recipient")
 	}
+
+	// The shared store invariant still rejects a command with no text, media, or
+	// rich payload; accepting rich-only must not make truly empty rows possible.
+	_, err = messages.SendPrivateText(ctx, domain.SendPrivateTextRequest{
+		SenderUserID:    sender.ID,
+		RecipientUserID: recipient.ID,
+		RandomID:        time.Now().UnixNano(),
+		Date:            int(time.Now().Unix()),
+	})
+	if !errors.Is(err, domain.ErrMessageEmpty) {
+		t.Fatalf("send empty private message err = %v, want ErrMessageEmpty", err)
+	}
 }

internal/store/postgres/message_send.go

@@ -107,8 +107,8 @@ func (s *MessageStore) sendPrivateTextOnce(ctx context.Context, req domain.SendP
 	if req.RandomID == 0 {
 		return domain.SendPrivateTextResult{}, fmt.Errorf("send private text: missing random id")
 	}
-	if req.Message == "" && req.Media.IsZero() {
-		return domain.SendPrivateTextResult{}, fmt.Errorf("send private text: empty message")
+	if !req.HasContent() {
+		return domain.SendPrivateTextResult{}, domain.ErrMessageEmpty
 	}
 	if req.Date == 0 {
 		req.Date = int(time.Now().Unix())