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/config/routers.go

@@ -24,7 +24,7 @@ func GetPublicConfig(c *gin.Context) {
 	ctx := c.Request.Context()
 	configs, err := model.ListVisibleSystemConfigs(ctx)
 	if err != nil {
-		c.JSON(http.StatusInternalServerError, response.Err(err.Error()))
+		response.AbortInternal(c, err.Error())
 		return
 	}