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>

.env.example

@@ -27,6 +27,14 @@ MAX_UPLOAD_SIZE_MB=500
 # Scale horizontally with: docker compose up --scale worker=N
 CELERY_WORKER_CONCURRENCY=8
 
+# MinIO (S3-compatible object storage for shared uploads)
+# MinIO is started automatically via docker-compose.yml
+MINIO_URL=http://minio:9000
+MINIO_USER=minioadmin
+MINIO_PASSWORD=minioadmin
+MINIO_BUCKET=uploads
+# MinIO console UI: http://localhost:9001 (user: minioadmin, password: minioadmin)
+
 # Blender (render-worker)
 # Blender >= 5.0.1 must be installed on the host at /opt/blender
 # The render-worker container mounts it read-only via volumes: - /opt/blender:/opt/blender:ro