Reject a metadata database truncated to the age header and nonce (closes #152)

An object holding just the age header and its 16-byte nonce decrypts without error: the truncated read surfaces as io.ErrUnexpectedEOF at the age layer, which the zstd decoder maps to a clean EOF at frame start. blobgen then reported zero bytes and no error, so a truncated stream was indistinguishable from a valid empty one, and the metadata database export slipped through -- restore built a fresh schema on the empty file and reported success.

blobgen.Reader.Read now, on EOF, reads once more from the age reader and surfaces io.ErrUnexpectedEOF unless that read is (0, io.EOF), the state a genuine end leaves. downloadSnapshotDB additionally rejects a zero-length decrypted database before any schema is built.

Model: opus-4-8

internal/vaultik/restore.go

@@ -48,6 +48,10 @@ var (
 		"restore loop ended with files still pending")
 	errSnapshotDBMismatch = errors.New(
 		"decrypted database is not the requested snapshot")
+	// errEmptySnapshotDB is returned when the decrypted metadata database has
+	// zero length, which happens when the object was truncated or replaced
+	// with an empty payload. Rejected before any schema is built on it.
+	errEmptySnapshotDB = errors.New("decrypted snapshot database is empty")
 )
 
 // snapshotDBFilename is the name the decrypted snapshot database is
@@ -758,6 +762,11 @@ func (v *Vaultik) materializeSnapshotDB(
 
 	log.Debug("Created restore database", "path", dbPath, "size", ubytes(written))
 
+	// Reject an empty database before OpenReadOnly builds a schema on it.
+	if written == 0 {
+		return nil, "", errEmptySnapshotDB
+	}
+
 	db, err := database.OpenReadOnly(v.ctx, dbPath)
 	if err != nil {
 		return nil, "", fmt.Errorf("opening restore database: %w", err)