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/risk_control/middleware.go

@@ -29,7 +29,7 @@ func RiskControlMiddleware() gin.HandlerFunc {
 
 		// 1. 限流背压检测（检测本地缓冲队列是否已满）
 		if IsBufferFull() {
-			c.AbortWithStatusJSON(http.StatusTooManyRequests, response.Err("系统繁忙，请稍后再试"))
+			response.AbortTooManyRequests(c, "系统繁忙，请稍后再试")
 			return
 		}