refactor(storage): update Backend.Put to return PutResult and encapsulate bucket mapping

- Update Backend.Put method signature in storage.go to return (PutResult, error).
- Adjust all backend implementations (local, oss, s3, webdav) to return a PutResult enclosing Key and Bucket.
- Refactor storeUploadFile in upload routers.go to extract key/bucket from PutResult, eliminating manual config bucket lookups.
- Remove the unused cfgBucket helper from storage_ops.go.
- Adjust storage_migration_task.go and tests to accommodate the updated method signature.

internal/storage/local.go

@@ -25,13 +25,13 @@ func newLocalBackend(cfg LocalConfig) (*localBackend, error) {
 	return &localBackend{root: root}, nil
 }
 
-func (b *localBackend) Put(_ context.Context, key string, body io.Reader, _ int64, _ string) (string, error) {
+func (b *localBackend) Put(_ context.Context, key string, body io.Reader, _ int64, _ string) (PutResult, error) {
 	path, err := b.path(key)
 	if err != nil {
-		return "", err
+		return PutResult{}, err
 	}
 	if err := os.MkdirAll(filepath.Dir(path), storageDirPerm); err != nil {
-		return "", err
+		return PutResult{}, err
 	}
 	file, err := os.OpenFile( //nolint:gosec // path is constrained to the configured storage root.
 		path,
@@ -39,18 +39,18 @@ func (b *localBackend) Put(_ context.Context, key string, body io.Reader, _ int6
 		storageFilePerm,
 	)
 	if err != nil {
-		return "", err
+		return PutResult{}, err
 	}
 	if _, err := io.Copy(file, body); err != nil {
 		_ = file.Close()
 		_ = os.Remove(path)
-		return "", err
+		return PutResult{}, err
 	}
 	if err := file.Close(); err != nil {
 		_ = os.Remove(path)
-		return "", err
+		return PutResult{}, err
 	}
-	return filepath.ToSlash(key), nil
+	return PutResult{Key: filepath.ToSlash(key)}, nil
 }
 
 func (b *localBackend) Get(_ context.Context, key string) (*Object, error) {