Persist workflow state transitions to DB, add migration, integration test, and fixes.

[jumping2000]

Summary

Persist workflow state transitions to DB, add migration, integration test, and fixes.

Key changes

• backend/app/workflows/execution/runner.py — persist states + guard fatal trigger
• backend/app/api/routes/workflow.py — use DB for start/get/approve/retry
• backend/app/db/models.py — owner_id made nullable
• backend/app/db/__init__.py — export session for tests
• backend/app/tests/test_workflow_persistence.py — integration test + ASGITransport fix
• backend/alembic/versions/20260523_001_initial_workflows.py — migration skeleton
• backend/pyproject.toml — hatch build config for editable installs

Verification

1. From backend/:

uv pip install -r requirements-dev.txt
uv run pytest -q

Expected: 34 passed (full suite)

Notes

• The branch contains additional test stubs and small fixes to make tests run in an isolated environment.
• Migration should be reviewed before applying to production Postgres (timestamps/defaults, UUID types).