Add S3 presigned URL direct upload (presign → PUT → confirm)

- Add presign endpoint: POST /api/v1/transactions/presign
- Add confirm endpoint: POST /api/v1/transactions/confirm
- Add generate_presigned_put_url() to MinIO service with public endpoint support
- Add public_endpoint field to MinioConfig
- Keep legacy /upload endpoint as fallback

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

config.example.yaml

@@ -3,6 +3,7 @@
 
 minio:
   endpoint: "https://minio.example.com:9000"
+  public_endpoint: "https://minio-public.example.com"
   bucket: "snapledger"
   access_key: "YOUR_MINIO_ACCESS_KEY"
   secret_key: "YOUR_MINIO_SECRET_KEY"