feat(api): showcase-workspace E1 — workspace persistence backbone (table + migration + DemoRunRequest fields + pipeline recording)

[w7-mgfcode]

Sub-issue of #389 (umbrella: showcase workspace — preserve, restore, and replay showcase runs). Foundation — blocks Epics #391, #392, #393.

Purpose

Workspace persistence backbone: showcase_workspace table (new app/features/demo/models.py, batch-slice precedent) + Alembic migration + additive Optional preservation/workspace fields on DemoRunRequest (HTTP + WS start frame, byte-compatible for legacy clients) + the pipeline records created run_ids, scenario_ids, batch_id, aliases, and artifact paths into the workspace row.

Sub-tasks

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

[w7-mgfcode]

Implemented and merged into dev via PR #394 (merge commit 0493192). Delivered: showcase_workspace table + migration 324a2fa37fcc, DemoRunRequest preservation/workspace_name fields, warn-and-continue workspace create/finalize hooks in run_pipeline, additive workspace_id on pipeline_complete/DemoRunResult, API_CONTRACTS notes. All five gates + demo integration suite green; Level-4 smoke verified (keep → completed row, ephemeral → no row, name-without-keep → 422). Unblocks E2 #391, E3 #392, E4 #393.