feat(api,ui): showcase-completion E6 — export bundle mvp (manifest + checksums + export endpoint/button)

[w7-mgfcode]

Sub-issue of #406 (umbrella: showcase workspace completion — the forecastlab control story). Parallel after Foundation (E1 #407).

Purpose

Export bundle MVP (export-only; import/restore is explicitly out of scope for this umbrella): a POST /demo/workspaces/{id}/export endpoint writing artifacts/showcase/<workspace>/ with a versioned manifest JSON (config, run summary, scenario plan snapshots, model-artifact references) and SHA-256 checksums, export validation, and an Export button on the workspace panel. Path handling reuses the LocalFSProvider traversal-prevention pattern with pathlib.Path.resolve() under an allow-listed root.

Sub-tasks

To be decomposed via issue-to-subtasks when this epic is picked up.

[w7-mgfcode]

Done — E6 export bundle merged to dev in #419 (merge commit 0381edb). Auto-close didn't fire because the PR landed on dev, not the default branch.

Shipped: POST /demo/workspaces/{id}/export writes a checksum-validated bundle (manifest + scenario-plan snapshots + checksums.sha256) under artifacts/showcase/<workspace_id>/; per-row Export button on the Saved-workspaces panel; soft references resolved in-process; model artifacts referenced (never copied); stateless (no migration). All five CI gates green. Import/restore remains out of scope per umbrella #406.