Add vitest coverage for workflows-db.ts run persistence

[aitrace-dev]

Context

packages/backend/src/workflows-db.ts persists workflow definitions and run history. It has no dedicated unit test. A focused vitest suite covering create / read / list / run-persistence would catch the 80% of bugs that matter.

Acceptance criteria

• New file packages/backend/src/workflows-db.test.ts
• Covers: create workflow, fetch by id, list workflows, record a run, list runs for a workflow, updating a workflow doesn't orphan its runs
• Uses an in-memory SQLite instance (no temp files)
• Passes with npm run test -w @granclaw/backend

Pointers

• packages/backend/src/workflows-db.test.ts (new)
• Existing patterns in packages/backend/src/*.test.ts

Difficulty

~2 hours. Good introduction to the SQLite layer.