Extract IMicCheckDbContext so DB access can be mocked with Moq instead
of the InMemory provider, per project testing guidelines. Add shared
async DbSet test doubles (TestSupport/) to back the mocks. Rewrite
HTTP-pipeline tests (WebApplicationFactory) to exercise controllers
and auth handlers directly instead of running the web server. Drop
DbContext/seeder tests that only made sense against a real provider,
and remove stale duplicate test files left over from a namespace move.
