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/oauth/handler_sources.go

@@ -0,0 +1,22 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package oauth
+
+import (
+	"net/http"
+
+	"github.com/Rain-kl/Wavelet/internal/common/response"
+	"github.com/gin-gonic/gin"
+)
+
+// GetLoginSources 获取可用登录源列表
+// @Summary 获取可用登录源
+// @Description 返回当前系统已启用的所有 OAuth 登录源，前端展示登录按钮列表时调用
+// @Tags oauth
+// @Produce json
+// @Success 200 {object} response.Any{data=[]oauth.AuthSourceView} "登录源列表"
+// @Router /api/v1/oauth/sources [get]
+func GetLoginSources(c *gin.Context) {
+	c.JSON(http.StatusOK, response.OK(activeLoginSources(c.Request.Context())))
+}