Define versioned staging blob and sidecar protocol

[niteshpurohit]

Objective

Define the shared on-disk contract for a staged file and its metadata.

Acceptance Criteria

Conceptually store:

opaque-id.ext

plus a versioned metadata sidecar containing:

• tenant;
• environment;
• destination;
• target;
• label;
• extension;
• multipart Content-Type;
• size;
• creation/publication time;
• expires_at;
• lifecycle/publication state; and
• format version.

Unknown newer incompatible sidecar versions fail safely.

Technical Considerations

• The sidecar is not a native C++ struct dump.
• No physical path is part of the external /files response contract.