fix: propagate errors in uploadBlobIfNeeded and listUniqueSnapshotIDs

- scanner.go: uploadBlobIfNeeded now returns (bool, error) instead of bool,
  preventing data loss where blobs were recorded in DB but upload failures
  were silently swallowed
- prune.go: listUniqueSnapshotIDs now returns ([]string, error) instead of
  []string, preventing incorrect orphan detection when listing errors occur

internal/snapshot/scanner.go

@@ -942,7 +942,11 @@ func (s *Scanner) handleBlobReady(blobWithReader *blob.BlobWithReader) error {
 	}
 
 	blobPath := fmt.Sprintf("blobs/%s/%s/%s", finishedBlob.Hash[:2], finishedBlob.Hash[2:4], finishedBlob.Hash)
-	blobExists := s.uploadBlobIfNeeded(ctx, blobPath, blobWithReader, startTime)
+	blobExists, err := s.uploadBlobIfNeeded(ctx, blobPath, blobWithReader, startTime)
+	if err != nil {
+		s.cleanupBlobTempFile(blobWithReader)
+		return fmt.Errorf("uploading blob %s: %w", finishedBlob.Hash, err)
+	}
 
 	if err := s.recordBlobMetadata(ctx, finishedBlob, blobExists, startTime); err != nil {
 		s.cleanupBlobTempFile(blobWithReader)
@@ -963,7 +967,7 @@ func (s *Scanner) handleBlobReady(blobWithReader *blob.BlobWithReader) error {
 }
 
 // uploadBlobIfNeeded uploads the blob to storage if it doesn't already exist, returns whether it existed
-func (s *Scanner) uploadBlobIfNeeded(ctx context.Context, blobPath string, blobWithReader *blob.BlobWithReader, startTime time.Time) bool {
+func (s *Scanner) uploadBlobIfNeeded(ctx context.Context, blobPath string, blobWithReader *blob.BlobWithReader, startTime time.Time) (bool, error) {
 	finishedBlob := blobWithReader.FinishedBlob
 
 	// Check if blob already exists (deduplication after restart)
@@ -972,14 +976,14 @@ func (s *Scanner) uploadBlobIfNeeded(ctx context.Context, blobPath string, blobW
 			"hash", finishedBlob.Hash, "size", humanize.Bytes(uint64(finishedBlob.Compressed)))
 		fmt.Printf("Blob exists: %s (%s, skipped upload)\n",
 			finishedBlob.Hash[:12]+"...", humanize.Bytes(uint64(finishedBlob.Compressed)))
-		return true
+		return true, nil
 	}
 
 	progressCallback := s.makeUploadProgressCallback(ctx, finishedBlob)
 
 	if err := s.storage.PutWithProgress(ctx, blobPath, blobWithReader.Reader, finishedBlob.Compressed, progressCallback); err != nil {
 		log.Error("Failed to upload blob", "hash", finishedBlob.Hash, "error", err)
-		return false
+		return false, fmt.Errorf("uploading blob to storage: %w", err)
 	}
 
 	uploadDuration := time.Since(startTime)
@@ -1004,7 +1008,7 @@ func (s *Scanner) uploadBlobIfNeeded(ctx context.Context, blobPath string, blobW
 		stats.BytesUploaded.Add(finishedBlob.Compressed)
 	}
 
-	return false
+	return false, nil
 }
 
 // makeUploadProgressCallback creates a progress callback for blob uploads