added revalidate cache feature

internal/hasher.go

@@ -0,0 +1,31 @@
+package steamimmich
+
+import (
+	"crypto/sha1"
+	"fmt"
+	"io"
+	"os"
+)
+
+func hashFiles(assets []AssetState) (map[int]string, error) {
+	results := make(map[int]string)
+
+	for i, p := range assets {
+		f, err := os.Open(p.FilePath)
+		if err != nil {
+			results[i] = ""
+			continue
+		}
+
+		h := sha1.New()
+		if _, err := io.Copy(h, f); err != nil {
+			f.Close()
+			return nil, err
+		}
+		f.Close()
+
+		results[i] = fmt.Sprintf("%x", h.Sum(nil))
+	}
+
+	return results, nil
+}