Adds 401, 403 demo-block, and cross-user isolation tests for:
GET /api/products/:id/next-story, GET /api/sprints/:id/tasks,
PATCH /api/stories/:id/tasks/reorder, POST /api/stories/:id/log,
POST /api/todos. Expands prisma mock to cover all required models.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>