feat(api,ui): add showcase workspace delete endpoint and panel action (#404)

[w7-mgfcode]

Summary

Adds workspace lifecycle deletion to the Showcase Workspace feature: DELETE /demo/workspaces/{workspace_id} (204 / RFC 7807 404) plus a confirmed Delete action on the /showcase Saved-workspaces panel. Deletion removes ONLY the showcase_workspace metadata row — the run's created objects (model runs, scenario plans, aliases, jobs, agent sessions, artifacts) are soft references and stay untouched. Docs swept (RUNBOOKS / DOMAIN_MODEL / API_CONTRACTS) to reflect the implemented workspace model with the remaining gaps marked explicitly out of scope.

Type of Change

• feat — new feature
• fix — bug fix
• feat! / fix! — breaking change (or BREAKING CHANGE: in the body)
• refactor — code restructure, no behavior change
• docs — documentation only
• test — test-only change
• chore / ci — tooling, CI, or repo hygiene (no version bump)

Checklist

Code Quality

• My code follows the project's coding standards (see CLAUDE.md)
• I have run uv run ruff check . --fix && uv run ruff format .
• I have run uv run mypy app/ && uv run pyright app/
• All type annotations are complete (no Any without justification)

Testing

• I have added tests that prove my fix/feature works
• All existing tests pass (uv run pytest -v) — 1977 unit + 16 demo integration; frontend 356 vitest, 61 files
• New tests follow existing patterns in app/*/tests/

Database

• No database changes required
• Migration added and tested (uv run alembic upgrade head on fresh DB)
• Migration is backward compatible

Documentation

• No documentation changes needed
• I have updated relevant documentation
• Docstrings follow Google style

Security

• No hardcoded secrets, credentials, or API keys
• Input validation is in place for user-provided data
• No SQL injection vulnerabilities (using SQLAlchemy ORM)

Testing Instructions

# Backend
uv run pytest app/features/demo -v -m "not integration"
docker compose up -d && uv run pytest app/features/demo -v -m integration

# Frontend
cd frontend && pnpm lint && pnpm test --run src/components/demo/WorkspacePanel.test.tsx src/hooks/use-workspaces.test.ts

# Manual: run a showcase with "Save as workspace" ticked, then Delete the row
# from the Saved-workspaces panel — confirm the dialog copy, the list refresh,
# and that /explorer/runs etc. still show the run the workspace referenced.

Key integration proof: test_delete_workspace_integration_keeps_created_objects creates a REAL agent session, soft-references it (plus a dangling run id) from a workspace, deletes the workspace, and asserts the session still exists.

Related Issues

Fixes #404. Relates to #389 (showcase-workspace umbrella), #390 (no-FK soft-reference design), #393 (E4 restore/replay).

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