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/shared/constants.go

@@ -0,0 +1,23 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+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
+	DetectContentBytes = 512              // http.DetectContentType 需要的最小字节数
+	UploadDirPerm      = 0755             // 上传目录权限
+	UploadFilePerm     = 0644             // 上传文件权限
+	ImageQualityLow    = "low"
+	ImageQualityMedium = "medium"
+	ImageQualityHigh   = "high"
+	ImageQualityOrigin = "origin"
+	StorageDriverLocal = string(storage.DriverLocal)
+	DefaultPublicUploadType = "avatar"
+	FileStatsTrendDays      = 7
+	MaxS3KeyLength          = 1024
+	AccessCacheTTL          = 5 // seconds; multiplied by time.Second at use site
+)