refactor: deduplicate sha1Hex/writeJSON/writeError into pkg/utils

- admin.go: replace local sha1Hex, sha256Hex, writeJSON, writeError with pkg/utils equivalents
- auth.go: replace local writeJSON with utils.WriteJSON; rewrite writeError as thin wrapper
- cas.go: remove local sha1Hex and unused writeJSON; use utils.SHA1Bytes
- pkg/utils.go: add WriteJSON, WriteError; reorder imports

internal/cas/cas.go

@@ -6,14 +6,14 @@
 package cas
 
 import (
-	"crypto/sha1"
 	"crypto/subtle"
-	"encoding/hex"
 	"net/http"
 	"os"
 	"path/filepath"
 	"strings"
 
+	"gitea.mrixs.me/Mrixs/MrixsCraft-server/pkg/utils"
+
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/config"
 	"gitea.mrixs.me/Mrixs/MrixsCraft-server/internal/database"
 )
@@ -148,7 +148,7 @@ func isValidHash(hash string) bool {
 // StoreFile writes data to the CAS directory structure.
 // Returns the SHA-1 hash of the stored data.
 func StoreFile(casDir string, data []byte) (string, error) {
-	hash := sha1Hex(data)
+	hash := utils.SHA1Bytes(data)
 	destDir := filepath.Join(casDir, hash[:2])
 	if err := os.MkdirAll(destDir, 0o755); err != nil {
 		return "", err
@@ -170,7 +170,7 @@ func FileExists(casDir, hash string) bool {
 // VerifyAndStore writes data to CAS only if the SHA-1 matches the expected hash.
 // This prevents corrupt or tampered uploads from being stored.
 func VerifyAndStore(casDir string, data []byte, expectedHash string) (string, error) {
-	got := sha1Hex(data)
+	got := utils.SHA1Bytes(data)
 	if subtle.ConstantTimeCompare([]byte(got), []byte(expectedHash)) != 1 {
 		return "", ErrHashMismatch
 	}
@@ -188,13 +188,3 @@ var ErrHashMismatch = errHashMismatch
 type hashMismatchError struct{}
 
 func (e *hashMismatchError) Error() string { return "SHA-1 hash mismatch" }
-
-func sha1Hex(data []byte) string {
-	h := sha1.Sum(data)
-	return hex.EncodeToString(h[:])
-}
-
-func writeJSON(w http.ResponseWriter, status int, v any) {
-	w.Header().Set("Content-Type", "application/json")
-	w.WriteHeader(status)
-}