feat(api,ui): showcase workspace lifecycle deletion (DELETE endpoint + panel action)

[w7-mgfcode]

Summary

The showcase-workspace initiative (#389) shipped keep/restore/replay, but showcase_workspace rows accumulate with no way to remove them — flagged as the top functional gap in the post-E5 audit and documented in RUNBOOKS as "no DELETE endpoint yet".

Scope

Backend

• DELETE /demo/workspaces/{workspace_id} — 204 on success, RFC 7807 404 when missing.
• Deletes ONLY the workspace metadata row. Created objects (model runs, scenario plans, aliases, jobs, agent sessions, artifacts) are soft references and stay untouched — symmetric with the no-FK design from feat(api): showcase-workspace E1 — workspace persistence backbone (table + migration + DemoRunRequest fields + pipeline recording) #390.

Frontend

• Per-row Delete action on the /showcase Saved-workspaces panel behind a confirmation dialog whose copy makes metadata-only deletion explicit; list refreshes on success; errors surface via the existing toast pattern.

Docs

• RUNBOOKS § Showcase workspace: lifecycle modes, deletion semantics, accumulation note, explicit out-of-scope list.
• DOMAIN_MODEL showcase_workspace aggregate: stored metadata, pipeline-run relationship, metadata-only-deletion invariant.
• API_CONTRACTS: new endpoint row.

Out of scope (explicit)

• Advanced seed config on /showcase
• Export bundles under artifacts/showcase/<workspace>/
• replayed_from provenance migration
• RAG event / approval-decision capture
• Full phase-level interactive configuration

Acceptance criteria

• 204 delete success + 404 missing route tests (unit + integration)
• Integration proof that soft-referenced objects survive deletion
• Frontend tests: delete button renders, confirmation copy is metadata-only-clear, success refreshes the list, failure surfaces an error
• No new migration; no weakened tests; all five validation gates green

[w7-mgfcode]

Shipped in PR #405 (merged to dev, bdf85f6). All acceptance criteria met:

• ✅ DELETE /demo/workspaces/{workspace_id} — 204 / RFC 7807 404; unit route tests + 2 integration tests (test_delete_workspace_integration_round_trip, test_delete_workspace_integration_keeps_created_objects)
• ✅ Soft-reference survival proven: a real agent session referenced in created_objects survives the workspace delete; a dangling run id deletes cleanly
• ✅ Frontend: panel Delete action + confirmation dialog with metadata-only copy, list refresh on success, toast on failure — 6 panel tests + 2 useDeleteWorkspace hook tests
• ✅ Docs swept: RUNBOOKS § Showcase workspace (lifecycle modes, deletion semantics, out-of-scope list), DOMAIN_MODEL showcase_workspace aggregate, API_CONTRACTS endpoint row
• ✅ No new migration; no weakened tests; CI green on PR feat(api,ui): add showcase workspace delete endpoint and panel action (#404) #405 (Lint & Format 16s, Type Check 1m20s, Test 1m55s, Migration Check 49s)

Out of scope (unchanged, documented): advanced seed config, export bundles, replayed_from provenance, RAG/approval capture, phase-level interactive config.