feat(backend): zip page index with natural sort, zip-slip rejection, content hash

backend/internal/bookfile/hash.go

@@ -0,0 +1,12 @@
+package bookfile
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+)
+
+func Hash(size, modTS int64) string {
+	sum := sha256.Sum256([]byte(fmt.Sprintf("%d:%d", size, modTS)))
+	return hex.EncodeToString(sum[:])[:16]
+}