feat(api,ui): showcase-workspace E4 — workspace restore/replay (list/load workspaces + replay through the run path)

[w7-mgfcode]

Sub-issue of #389 (umbrella: showcase workspace — preserve, restore, and replay showcase runs). Parallel after Foundation (E1 #390).

Purpose

Workspace restore/replay: list and load saved showcase_workspace rows (config reloads into the Showcase UI, replacing the localStorage-only FIFO-5 history for workspace runs), re-attach to the workspace's created objects (runs, plans, aliases, artifacts), and replay a recorded configuration with the same seed/preset through the existing run path without 409/500 (replay blockers #146/#324 are verifiably fixed). Restore-vs-replay semantics (re-run vs re-attach) are designed in this epic's PRP.

Sub-tasks

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

[w7-mgfcode]

Shipped in PR #397 (merged into dev): GET /demo/workspaces + /{id}, Showcase workspace controls (seed/keep/name), server-backed WorkspacePanel with Load/Replay, WorkspaceArtifactsPanel deep links, ephemeral-only RunHistoryStrip, and the same-config replay regression test (both runs green, distinct workspace ids — #146/#324 guard).