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>

.gitignore

@@ -3,7 +3,8 @@ node_modules/
 .env.local
 .DS_Store
 *.log
-core
+# core dump files (not directories named 'core')
+/core
 /blender-renderer/core
 
 # Python cache