feat: per-round advancement selection, email preview, Docker/auth fixes

- Bulk notification dialog: per-round checkboxes (default none selected),
  selected count badge, "Preview Email" button with rendered iframe
- Backend: roundIds filter on sendBulkPassedNotifications, new
  previewAdvancementEmail query
- Docker: add external MinIO network so app container can reach MinIO
- File router: try/catch on getPresignedUrl with descriptive error
- Auth: custom NextAuth logger suppresses CredentialsSignin stack traces

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docker/docker-compose.yml

@@ -50,6 +50,7 @@ services:
         condition: service_healthy
     networks:
       - mopc-network
+      - minio-external
     healthcheck:
       test: ["CMD", "node", "-e", "fetch('http://localhost:7600/api/health').then(r=>{if(!r.ok)throw r;process.exit(0)}).catch(()=>process.exit(1))"]
       interval: 30s
@@ -82,3 +83,6 @@ volumes:
 networks:
   mopc-network:
     driver: bridge
+  minio-external:
+    external: true
+    name: minio_mopc-minio