fix: correct HTTP status codes and error handling in remaining handlers

- createPlaylistHandler: 200 → 201 Created, service errors → 500
- createSiteHandler: service/DB errors → 500 (was 400)
- saveStorageConfigHandler: service errors → 500 (was 400)
- SchedulerHandler.RunTask: task errors → 500 (was 400 with ErrInternal)
- aria2StatsHandler: service errors → 500 (was 400)
- organizeMediaHandler: service errors → 500 (was 400)
- testDownloadClientHandler: test failure → 200 with ok:false (was 400)
- testStorageConfigHandler: test failure → 200 with ok:false (was 400)

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

internal/handler/playback.go

@@ -109,10 +109,10 @@ func createPlaylistHandler(svc *service.Container) gin.HandlerFunc {
 			c.Request.Context(), uid.(string), req.Name, req.IsPublic,
 		)
 		if err != nil {
-			c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+			c.JSON(http.StatusInternalServerError, gin.H{"error": err.Error()})
 			return
 		}
-		c.JSON(http.StatusOK, pl)
+		c.JSON(http.StatusCreated, pl)
 	}
 }