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) {

internal/storage/local_test.go

@@ -19,12 +19,12 @@ func TestLocalBackendRoundTrip(t *testing.T) {
 	const key = "uploads/2026/06/13/test.txt"
 	const content = "wavelet storage"
 
-	storedKey, err := backend.Put(ctx, key, bytes.NewBufferString(content), int64(len(content)), "text/plain")
+	storedResult, err := backend.Put(ctx, key, bytes.NewBufferString(content), int64(len(content)), "text/plain")
 	if err != nil {
 		t.Fatalf("Put(%q) returned error: %v", key, err)
 	}
-	if storedKey != key {
-		t.Errorf("Put(%q) key = %q, want %q", key, storedKey, key)
+	if storedResult.Key != key {
+		t.Errorf("Put(%q) key = %q, want %q", key, storedResult.Key, key)
 	}
 
 	object, err := backend.Get(ctx, key)

internal/storage/oss.go

@@ -35,7 +35,7 @@ func newOSSBackend(cfg ObjectConfig) (*ossBackend, error) {
 	}, nil
 }
 
-func (b *ossBackend) Put(ctx context.Context, key string, body io.Reader, _ int64, _ string) (string, error) {
+func (b *ossBackend) Put(ctx context.Context, key string, body io.Reader, _ int64, _ string) (PutResult, error) {
 	key = b.key(key)
 	_, err := b.client.PutObject(ctx, &oss.PutObjectRequest{
 		Bucket: oss.Ptr(b.bucket),
@@ -43,9 +43,9 @@ func (b *ossBackend) Put(ctx context.Context, key string, body io.Reader, _ int6
 		Body:   body,
 	})
 	if err != nil {
-		return "", fmt.Errorf("put OSS object: %w", err)
+		return PutResult{}, fmt.Errorf("put OSS object: %w", err)
 	}
-	return key, nil
+	return PutResult{Key: key, Bucket: b.bucket}, nil
 }
 
 func (b *ossBackend) Get(ctx context.Context, key string) (*Object, error) {

internal/storage/s3.go

@@ -56,7 +56,7 @@ func newR2Backend(ctx context.Context, cfg ObjectConfig) (*s3Backend, error) {
 	return newS3Backend(ctx, cfg)
 }
 
-func (b *s3Backend) Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) (string, error) {
+func (b *s3Backend) Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) (PutResult, error) {
 	key = b.key(key)
 	_, err := b.client.PutObject(ctx, &s3.PutObjectInput{
 		Bucket:        aws.String(b.bucket),
@@ -66,9 +66,9 @@ func (b *s3Backend) Put(ctx context.Context, key string, body io.Reader, size in
 		ContentType:   aws.String(contentType),
 	})
 	if err != nil {
-		return "", fmt.Errorf("put S3 object: %w", err)
+		return PutResult{}, fmt.Errorf("put S3 object: %w", err)
 	}
-	return key, nil
+	return PutResult{Key: key, Bucket: b.bucket}, nil
 }
 
 func (b *s3Backend) Get(ctx context.Context, key string) (*Object, error) {

internal/storage/storage.go

@@ -32,9 +32,15 @@ type Object struct {
 	ContentType   string
 }
 
+// PutResult describes the result of a successful Put operation.
+type PutResult struct {
+	Key    string
+	Bucket string
+}
+
 // Backend defines storage operations used by the upload domain.
 type Backend interface {
-	Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) (string, error)
+	Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) (PutResult, error)
 	Get(ctx context.Context, key string) (*Object, error)
 	Delete(ctx context.Context, key string) error
 	Test(ctx context.Context) error
@@ -173,11 +179,11 @@ type functionBackend struct {
 	delete func(context.Context, string) error
 }
 
-func (b *functionBackend) Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) (string, error) {
+func (b *functionBackend) Put(ctx context.Context, key string, body io.Reader, size int64, contentType string) (PutResult, error) {
 	if err := b.put(ctx, key, body, size, contentType); err != nil {
-		return "", err
+		return PutResult{}, err
 	}
-	return key, nil
+	return PutResult{Key: key}, nil
 }
 
 func (b *functionBackend) Get(ctx context.Context, key string) (*Object, error) {

internal/storage/webdav.go

@@ -28,17 +28,17 @@ func newWebDAVBackend(cfg WebDAVConfig) (*webDAVBackend, error) {
 	}, nil
 }
 
-func (b *webDAVBackend) Put(_ context.Context, key string, body io.Reader, size int64, _ string) (string, error) {
+func (b *webDAVBackend) Put(_ context.Context, key string, body io.Reader, size int64, _ string) (PutResult, error) {
 	key = b.key(key)
 	if dir := path.Dir(key); dir != "." && dir != "/" {
 		if err := b.client.MkdirAll(dir, storageDirPerm); err != nil {
-			return "", fmt.Errorf("create WebDAV directory: %w", err)
+			return PutResult{}, fmt.Errorf("create WebDAV directory: %w", err)
 		}
 	}
 	if err := b.client.WriteStreamWithLength(key, body, size, storageFilePerm); err != nil {
-		return "", fmt.Errorf("put WebDAV object: %w", err)
+			return PutResult{}, fmt.Errorf("put WebDAV object: %w", err)
 	}
-	return key, nil
+	return PutResult{Key: key}, nil
 }
 
 func (b *webDAVBackend) Get(_ context.Context, key string) (*Object, error) {