Write a test plan for this spec.

Include:
- Unit tests: key pure logic, boundary conditions
- Integration tests: DB/API boundaries, mocks vs real dependencies
- E2E tests: critical user journeys (if applicable)
- Non-functional tests: performance, rate limits, resiliency (if relevant)

For each test group:
- what to test,
- what to mock/stub,
- minimal set that must pass for “done”.

If the spec is brownfield, include regression coverage suggestions.