Add DUMMY_MARK_READ env to control marking backfills as read

Annoying this is an env var but not really any other way to do it except
config which would be equally annoying.

pkg/connector/backfill.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"math"
 	"math/rand"
+	"os"
 	"time"
 
 	"maunium.net/go/mautrix/bridgev2"
@@ -65,5 +66,10 @@ func (dc *DummyClient) FetchMessages(ctx context.Context, fetchParams bridgev2.F
 
 	// always claim we have more until timelimit is hit
 	resp.HasMore = true
+
+	if os.Getenv("DUMMY_MARK_READ") != "" {
+		resp.MarkRead = true
+	}
+
 	return
 }