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/router/middlewares.go

@@ -7,7 +7,6 @@ package router
 
 import (
 	"context"
-	"errors"
 	"net/http"
 	"strconv"
 	"strings"
@@ -106,29 +105,7 @@ func corsMiddleware() gin.HandlerFunc {
 	}
 }
 
-// errorHandlerMiddleware 捕获 c.Errors 并统一格式化为 JSON 返回给客户端，同时将其记录到 Span 异常中
+// errorHandlerMiddleware 委托给 response.ErrorHandlerMiddleware，保持路由层单一入口。
 func errorHandlerMiddleware() gin.HandlerFunc {
-	return func(c *gin.Context) {
-		c.Next()
-
-		if len(c.Errors) > 0 {
-			err := c.Errors.Last().Err
-			span := trace.SpanFromContext(c.Request.Context())
-
-			// 1. 如果有活跃的 Span，将错误信息记录到 Trace 中，并把 Span 状态置为 Error
-			if span.IsRecording() {
-				span.RecordError(err)
-				span.SetStatus(codes.Error, err.Error())
-			}
-
-			// 2. 将错误转化为统一的 JSON 格式响应给客户端
-			var apiErr *response.APIError
-			if errors.As(err, &apiErr) {
-				c.JSON(apiErr.Code, response.Err(apiErr.Msg))
-			} else {
-				// 兜底策略：未知的系统级错误
-				c.JSON(http.StatusInternalServerError, response.Err("内部系统错误"))
-			}
-		}
-	}
+	return response.ErrorHandlerMiddleware()
 }