F2.1e — Add the S3-compatible storage seam with MinIO

[Flow-Fly]

Parent: #133. Grandparent: #132 / #72. Depends on: #265.

Goal

Prove byte-exact project-blob storage behind a small S3-compatible adapter using MinIO locally, without provisioning cloud resources.

Scope

• Define a narrow blob-storage interface owned by the server.
• Implement the S3-compatible adapter using owner-approved configuration.
• Add MinIO to local Docker Compose.
• Add deterministic put/get/delete integration tests using isolated test keys.
• Extend server health reporting with explicit storage availability.
• Document endpoint, region, bucket, and credential configuration.

Out of scope

• No Scaleway bucket creation, production credentials, project API, auth, retention policy, or garbage collection.
• No client changes.

Acceptance

• Arbitrary project bytes round-trip byte-identically through MinIO.
• Test cleanup is reliable and cannot target production configuration.
• Missing storage configuration fails clearly.
• The adapter does not expose AWS SDK details to route handlers.

Risk and routing

Risk: medium because this establishes the blob-storage boundary.
Worker-ready: no; refresh after the local server skeleton lands.
Review need: careful configuration and test-isolation review.