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>

.env.example

@@ -35,6 +35,18 @@ JWT_SECRET=your-secure-jwt-secret-change-in-production
 # Defaults to http://localhost:3000,http://localhost:5173 when unset
 # CORS_ORIGINS=https://sparc.example.com,https://app.example.com
 
+# ---- Storage ----
+
+# Backend for patent PDF storage: "local" (default) or "s3"
+STORAGE_BACKEND=local
+
+# S3/MinIO settings (only used when STORAGE_BACKEND=s3)
+# S3_BUCKET=sparc-patents
+# S3_ENDPOINT_URL=http://localhost:9000
+# AWS_ACCESS_KEY_ID=minioadmin
+# AWS_SECRET_ACCESS_KEY=minioadmin
+# To start MinIO locally: docker compose --profile s3 up -d minio
+
 # ---- Cache ----
 
 # When USE_CACHE=true: check database for cached responses before making API calls