Spins up a MinIO container (Testcontainers GenericContainer) alongside the existing PostgresContainerConfig, uploads a sample PDF, runs the real ThumbnailService, and reads the resulting JPEG back from the object store. Catches S3 signing / path-style access issues a mocked S3Client wouldn't — justifies the CI cost (~45s) per walkthrough T9b. Refs #307 Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>