fix: update plane backend for new stable image requirements

makeplane/plane-backend:stable now requires:
- AMQP_URL: Celery broker URL (defaults to amqp://localhost, broken)
  → set to redis://plane-redis:6379/ to reuse existing Redis
- GUNICORN_WORKERS: must be set explicitly (empty string causes crash)
  → set to 2

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

roles/services/templates/docker-compose.yml.j2

@@ -185,6 +185,7 @@ services:
     environment:
       - DATABASE_URL=postgresql://plane:${PLANE_DB_PASSWORD}@plane-db:5432/plane
       - REDIS_URL=redis://plane-redis:6379/
+      - AMQP_URL=redis://plane-redis:6379/
       - SECRET_KEY=${PLANE_SECRET_KEY}
       - DEBUG=0
       - DJANGO_SETTINGS_MODULE=plane.settings.production
@@ -198,6 +199,7 @@ services:
       - AWS_S3_BUCKET_NAME=uploads
       - MINIO_ROOT_USER=plane-minio
       - MINIO_ROOT_PASSWORD=${PLANE_MINIO_PASSWORD}
+      - GUNICORN_WORKERS=2
     labels:
       - "traefik.enable=true"
       - "traefik.http.routers.plane-api.rule=Host(`{{ domain_plane }}`) && (PathPrefix(`/api/`) || PathPrefix(`/auth/`))"
@@ -219,6 +221,7 @@ services:
     environment:
       - DATABASE_URL=postgresql://plane:${PLANE_DB_PASSWORD}@plane-db:5432/plane
       - REDIS_URL=redis://plane-redis:6379/
+      - AMQP_URL=redis://plane-redis:6379/
       - SECRET_KEY=${PLANE_SECRET_KEY}
       - DEBUG=0
       - DJANGO_SETTINGS_MODULE=plane.settings.production
@@ -243,6 +246,7 @@ services:
     environment:
       - DATABASE_URL=postgresql://plane:${PLANE_DB_PASSWORD}@plane-db:5432/plane
       - REDIS_URL=redis://plane-redis:6379/
+      - AMQP_URL=redis://plane-redis:6379/
       - SECRET_KEY=${PLANE_SECRET_KEY}
       - DEBUG=0
       - DJANGO_SETTINGS_MODULE=plane.settings.production