feat(security): use bcrypt hashes and safe paths for boxes

- Replace legacy salted password hashing with bcrypt and store hash alg
- Accept existing bcrypt hashes while keeping legacy verification fallback
- Validate box IDs and use SafeChildPath for box/file operations to prevent traversal
- Refactor download flow to share zip writer logic and correctly handle one-time deletes and optional renew-on-download only after a successful zip writefeat(security): use bcrypt hashes and safe paths for boxes

- Replace legacy salted password hashing with bcrypt and store hash alg
- Accept existing bcrypt hashes while keeping legacy verification fallback
- Validate box IDs and use SafeChildPath for box/file operations to prevent traversal
- Refactor download flow to share zip writer logic and correctly handle one-time deletes and optional renew-on-download only after a successful zip write

lib/helpers/paths_test.go

@@ -0,0 +1,20 @@
+package helpers
+
+import (
+	"path/filepath"
+	"testing"
+)
+
+func TestSafeChildPathRejectsTraversalAndAbsolutePaths(t *testing.T) {
+	parent := filepath.Join(t.TempDir(), "parent")
+
+	if _, ok := SafeChildPath(parent, "../outside.txt"); ok {
+		t.Fatal("expected traversal to be rejected")
+	}
+	if _, ok := SafeChildPath(parent, filepath.Join(string(filepath.Separator), "tmp", "outside.txt")); ok {
+		t.Fatal("expected absolute path to be rejected")
+	}
+	if path, ok := SafeChildPath(parent, "inside.txt"); !ok || path != filepath.Join(parent, "inside.txt") {
+		t.Fatalf("expected safe child path, got path=%q ok=%v", path, ok)
+	}
+}