feat(A4): add MinIO service + storage abstraction (core/storage.py)

- Add MinIO service to docker-compose.yml (port 9000 API, 9001 console)
- Add minio-data volume for persistent object storage
- Create backend/app/core/storage.py: MinIOStorage + LocalStorage abstraction
  - MinIOStorage: boto3-based, auto-creates bucket, upload/download/exists/delete/presign
  - LocalStorage: fallback for dev (UPLOAD_DIR filesystem, backward compat)
  - get_storage() singleton: auto-selects based on MINIO_URL env var
- Add MINIO_URL/USER/PASSWORD/BUCKET env vars to all service definitions
- backend/pyproject.toml: docker>=6.1.0 → boto3>=1.34.0
- Add docker-compose.worker.yml: external render-worker for remote machines
- Fix .gitignore: 'core' rule was too broad, now only matches root /core dump
- Update .env.example: MinIO connection vars documented

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

docker-compose.worker.yml

@@ -0,0 +1,49 @@
+# External render-worker configuration.
+#
+# Use this compose file on remote machines (GPU nodes, NAS, cloud VMs) that
+# only need Redis + MinIO access — no database, no backend, no frontend.
+#
+# Usage on remote machine:
+#   export REDIS_URL=redis://main-server:6379/0
+#   export MINIO_URL=http://main-server:9000
+#   export MINIO_USER=minioadmin
+#   export MINIO_PASSWORD=minioadmin
+#   docker compose -f docker-compose.worker.yml up -d
+#
+# Requirements on remote machine:
+#   - Blender >= 5.0.1 installed at /opt/blender (or set BLENDER_BIN)
+#   - Docker + GPU drivers (NVIDIA recommended)
+
+services:
+  render-worker:
+    image: schaefflerautomat-render-worker:latest
+    # Or build locally: build: { context: ./render-worker, dockerfile: Dockerfile }
+    environment:
+      - REDIS_URL=${REDIS_URL:?Set REDIS_URL to the main server Redis URL}
+      - POSTGRES_HOST=${POSTGRES_HOST:?Set POSTGRES_HOST to the main server DB host}
+      - POSTGRES_DB=${POSTGRES_DB:-schaeffler}
+      - POSTGRES_USER=${POSTGRES_USER:-schaeffler}
+      - POSTGRES_PASSWORD=${POSTGRES_PASSWORD:?Set POSTGRES_PASSWORD}
+      - POSTGRES_PORT=${POSTGRES_PORT:-5432}
+      - MINIO_URL=${MINIO_URL:?Set MINIO_URL to the main server MinIO URL}
+      - MINIO_USER=${MINIO_USER:-minioadmin}
+      - MINIO_PASSWORD=${MINIO_PASSWORD:-minioadmin}
+      - MINIO_BUCKET=${MINIO_BUCKET:-uploads}
+      - UPLOAD_DIR=/tmp/render-cache
+      - BLENDER_BIN=${BLENDER_BIN:-/opt/blender/blender}
+      - RENDER_SCRIPTS_DIR=/render-scripts
+      - JWT_SECRET_KEY=${JWT_SECRET_KEY:?Set JWT_SECRET_KEY}
+    volumes:
+      - /opt/blender:/opt/blender:ro
+      - render-cache:/tmp/render-cache
+    deploy:
+      replicas: ${RENDER_WORKER_REPLICAS:-1}
+      resources:
+        reservations:
+          devices:
+            - driver: nvidia
+              count: 1
+              capabilities: [gpu, compute, utility, graphics]
+
+volumes:
+  render-cache: