feat: add S3/MinIO object storage support for patent PDFs

Introduce a StorageBackend abstraction (local filesystem and S3) for
patent PDF storage. When STORAGE_BACKEND=s3, PDFs are read/written via
boto3 to an S3-compatible bucket instead of the local filesystem.

- Add SPARC/storage.py with LocalStorageBackend and S3StorageBackend
- Update serp_api.py save_patents and parse_patent_pdf to use storage
- Add storage config vars to config.py and .env.example
- Add optional MinIO service to docker-compose.yml (--profile s3)
- Add boto3 to requirements.txt

Closes leeworks-agents/SPARC#38

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

requirements.txt

@@ -15,3 +15,4 @@ pandas
 bcrypt
 PyJWT
 slowapi
+boto3