[P1-07] Add external artifact storage and lifecycle management

[erikhoward]

Goal

Support large and binary workflow data without putting it directly into envelopes, events, or API responses.

Scope

• Define a pluggable artifact/blob store with content-addressed or stable references.
• Enforce content type, size, retention, authorization, and download policies.
• Store metadata and references in run state/events while avoiding unnecessary payload duplication.
• Add cleanup, expiration, garbage collection, and export behavior.

Acceptance criteria

• Large artifacts do not exhaust runtime memory or event-store limits.
• Artifact access is tenant- and run-authorized.
• Expired/unreferenced artifacts are cleaned up safely.
• Failure and partial-upload behavior is resumable or explicitly terminal.

Priority

P1 — high-value operations enhancement.

See the PetalFlow 1.0.0 Readiness Report for the audit context.