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>

docker-compose.yml

@@ -52,6 +52,29 @@ services:
       - ./patents:/app/patents
     restart: unless-stopped
 
+  # Optional: MinIO for S3-compatible local object storage
+  # Enable by setting STORAGE_BACKEND=s3 in .env
+  minio:
+    image: minio/minio:latest
+    container_name: sparc-minio
+    command: server /data --console-address ":9001"
+    environment:
+      MINIO_ROOT_USER: ${AWS_ACCESS_KEY_ID:-minioadmin}
+      MINIO_ROOT_PASSWORD: ${AWS_SECRET_ACCESS_KEY:-minioadmin}
+    ports:
+      - "9000:9000"
+      - "9001:9001"
+    volumes:
+      - minio_data:/data
+    healthcheck:
+      test: ["CMD", "mc", "ready", "local"]
+      interval: 10s
+      timeout: 5s
+      retries: 3
+    restart: unless-stopped
+    profiles:
+      - s3
+
   dashboard:
     build: ./frontend
     container_name: sparc-dashboard
@@ -63,3 +86,4 @@ services:
 
 volumes:
   postgres_data:
+  minio_data: