refactor(api): unify error handling and split upload/oauth god modules

Replace c.JSON(200, response.Err) and middleware gin.H bypasses with
response.Abort* helpers so errors flow through Gin Error chain and
ErrorHandlerMiddleware for OTel trace correlation.

Split oauth/sources.go into domain-focused files and decompose upload
into handler/filesrv/stats/task/cache/storage/util subpackages with a
root facade preserving existing import paths.

internal/apps/upload/storage/storage_ops.go

@@ -0,0 +1,67 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package storage
+
+import (
+	"context"
+	"fmt"
+
+	"github.com/Rain-kl/Wavelet/internal/model"
+	"github.com/Rain-kl/Wavelet/internal/storage"
+	"github.com/Rain-kl/Wavelet/pkg/logger"
+)
+
+// ReadOnly checks if the storage system is in read-only maintenance mode.
+func ReadOnly(ctx context.Context) bool {
+	state := LoadMigrationAccessState(ctx)
+	if state.LoadErr != nil {
+		logger.ErrorF(ctx, "读取存储维护状态失败: %v", state.LoadErr)
+		return true
+	}
+	return state.ReadOnly
+}
+
+// OpenStoredObject opens a stored upload object from its configured backend.
+func OpenStoredObject(ctx context.Context, upload *model.Upload) (*storage.Object, error) {
+	driver := storage.Driver(upload.StorageDriver)
+	if driver == "" {
+		driver = storage.DriverLocal
+	}
+	backend, err := backendForStoredDriver(ctx, driver)
+	if err != nil {
+		return nil, err
+	}
+	return backend.Get(ctx, upload.FilePath)
+}
+
+func backendForStoredDriver(ctx context.Context, driver storage.Driver) (storage.Backend, error) {
+	backend, err := storage.ForDriver(ctx, driver)
+	if err == nil {
+		return backend, nil
+	}
+
+	target, ok, targetErr := CurrentMigrationTargetConfig(ctx)
+	if targetErr != nil {
+		return nil, targetErr
+	}
+	if ok && target.Driver == driver {
+		return storage.NewBackend(ctx, target, driver)
+	}
+	return nil, fmt.Errorf("storage configuration for driver %q is unavailable", driver)
+}
+
+// CurrentMigrationTargetConfig returns the pending migration target config when available.
+func CurrentMigrationTargetConfig(ctx context.Context) (storage.Config, bool, error) {
+	state := LoadMigrationAccessState(ctx)
+	if state.LoadErr != nil {
+		return storage.Config{}, false, state.LoadErr
+	}
+	if state.TargetErr != nil {
+		return storage.Config{}, false, state.TargetErr
+	}
+	if !state.HasTarget {
+		return storage.Config{}, false, nil
+	}
+	return state.Target, true, nil
+}