fix(backend): отдавать контрольную сумму оригинала в ответе на загрузку

Ответ на загрузку и публикуемое событие теперь несут хеш сохранённого
объекта, чтобы клиент синхронизации мог сверить принятые байты со своим
файлом и только после этого удалять прежнюю версию элемента.

backend/internal/app/photo_checksum_test.go

@@ -2,7 +2,9 @@ package app
 
 import (
 	"context"
+	"crypto/sha256"
 	"encoding/json"
+	"fmt"
 	"net/http"
 	"testing"
 
@@ -70,3 +72,36 @@ func TestPhotoResponsesExposeOriginalChecksum(t *testing.T) {
 		t.Fatalf("list original_checksum = %+v, want one photo with sha256-photo-a", list.Photos)
 	}
 }
+
+// The upload response is photo data too: it must carry the stored original's
+// checksum, which a synchronization client uses to confirm what the server
+// actually accepted before it drops an earlier version of the same element.
+func TestUploadResponseExposesOriginalChecksum(t *testing.T) {
+	ctx := context.Background()
+	r, token, uid, repos := newAvatarServer(t)
+
+	album := &models.Album{ID: auth.NewID(), OwnerID: uid, Title: "A"}
+	if err := repos.Albums.Create(ctx, album); err != nil {
+		t.Fatalf("create album: %v", err)
+	}
+	if err := repos.Members.Add(ctx, album.ID, uid, "editor", nil); err != nil {
+		t.Fatalf("add owner member: %v", err)
+	}
+
+	img := makeTestPNG(t)
+	body, contentType := multipartFile(t, "file", "c.png", img)
+	rec := doAuthed(r, http.MethodPost, "/api/albums/"+album.ID+"/photos", body, contentType, token)
+	if rec.Code != http.StatusCreated {
+		t.Fatalf("upload = %d body=%s", rec.Code, rec.Body.String())
+	}
+	var out struct {
+		Photos []models.Photo `json:"photos"`
+	}
+	if err := json.Unmarshal(rec.Body.Bytes(), &out); err != nil {
+		t.Fatalf("decode upload: %v", err)
+	}
+	want := fmt.Sprintf("%x", sha256.Sum256(img))
+	if len(out.Photos) != 1 || out.Photos[0].OriginalChecksum != want {
+		t.Fatalf("upload original_checksum = %+v, want %q", out.Photos, want)
+	}
+}

backend/internal/photos/service.go

@@ -216,6 +216,9 @@ func (s *Service) Upload(ctx context.Context, in UploadInput) (*models.Photo, er
 		Height:     height,
 		Checksum:   hex.EncodeToString(hasher.Sum(nil)),
 	}
+	// The response and the published event carry the checksum the object ledger
+	// holds, so a client can match its local file against the stored bytes.
+	p.OriginalChecksum = obj.Checksum
 
 	// The metadata layer is the ledger of record: the row is written first,
 	// then the bytes are stored, then the object is activated.