chore(testnet): bump backend image to v10.2.1.commit.2be39f50

The previous build (db0a69a) had mismatched mix.exs version (10.0.6),
causing /app/releases/10.0.6/runtime.exs to reference a non-existent
config_helper.exs. This rebuild has correct version files restored
from upstream v10.2.1, releases now live at /app/releases/10.2.1/.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: JOY

docker-compose/docker-compose-testnet.yml

@@ -82,7 +82,7 @@ services:
       - db
       - redis-db
       - avago
-    image: ghcr.io/dos/doscan:10.2.1.commit.db0a69af
+    image: ghcr.io/dos/doscan:10.2.1.commit.2be39f50
     pull_policy: always
     restart: always
     stop_grace_period: 5m