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/testhelper/gin.go

@@ -0,0 +1,20 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package testhelper
+
+import (
+	"github.com/Rain-kl/Wavelet/internal/common/response"
+	"github.com/gin-gonic/gin"
+)
+
+// NewTestGinEngine 创建带 ErrorHandlerMiddleware 的 Gin 引擎，与生产环境错误响应行为一致。
+func NewTestGinEngine(middlewares ...gin.HandlerFunc) *gin.Engine {
+	gin.SetMode(gin.TestMode)
+	r := gin.New()
+	r.Use(response.ErrorHandlerMiddleware())
+	for _, middleware := range middlewares {
+		r.Use(middleware)
+	}
+	return r
+}