feat(api,db): showcase-completion E1 — workspace metadata & provenance backbone (migration + jsonb story slots + patch lifecycle endpoint)

[w7-mgfcode]

Sub-issue of #406 (umbrella: showcase workspace completion — the forecastlab control story). Foundation — blocks Epics #408, #409, #410, #411, #412.

Purpose

Workspace metadata & provenance backbone: one Alembic migration extending showcase_workspace with lifecycle + provenance columns (replayed_from_workspace_id, archived, pinned, notes, tags, config_schema_version) and documented JSONB story slots (seed_overrides, user_scope, approval_events, rag_events, job_ids, phase_summaries), plus a PATCH /demo/workspaces/{id} lifecycle endpoint (rename/notes/tags/archive/pin) and Pydantic schema surface. Replay writes replayed_from_workspace_id. Every Parallel epic writes into or reads from this surface, so it ships first. All changes additive; migration applies + downgrades cleanly on a fresh DB.

Sub-tasks

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

[w7-mgfcode]

Shipped in PR #414 (merged to dev @ 3611908): migration d45cf40dfe47 (12 columns + tags GIN + replayed-from index), PATCH /demo/workspaces/{id} lifecycle endpoint, replay provenance recording end-to-end (UI start frame → create_workspace), additive response fields, story-slot schemas documented in DOMAIN_MODEL. All gates + integration + Level-4 live smoke green.