TestChannelStore sub-tests create channels, members, and team members
using fake TeamIds and UserIds (model.NewId() for non-existent rows).
These records are left in the database and cause integrity tests
(TestCheck*) running in the same binary to fail their full-table scans.
Register a t.Cleanup on TestChannelStore that purges the affected
tables entirely. A blanket purge is safe: the schema enforces no FK
constraints, and every test suite creates its own data independently.