refactor(storage): drop per-upload storage_driver, use storage_config as single source

Remove w_uploads.storage_driver and route all read/write/delete paths through
storage.Active() backed by storage_config.driver. Block direct driver switches
when uploads exist; require migration task instead. Simplify migration to
cursor-based file_path iteration without per-row driver updates.

internal/apps/upload/shared/constants.go

@@ -3,8 +3,6 @@
 
 package shared
 
-import "github.com/Rain-kl/Wavelet/internal/storage"
-
 // Upload size, path, media quality, and cache constants shared across subpackages.
 const (
 	MaxUploadSize      = 32 * 1024 * 1024 // 32MB
@@ -15,7 +13,6 @@ const (
 	ImageQualityMedium = "medium"
 	ImageQualityHigh   = "high"
 	ImageQualityOrigin = "origin"
-	StorageDriverLocal = string(storage.DriverLocal)
 	DefaultPublicUploadType = "avatar"
 	FileStatsTrendDays      = 7
 	MaxS3KeyLength          = 1024