Remove module-level engine/session replacement that affected all tests
including e2e. The test engine is now only used via the session fixture,
so e2e tests keep using the real DB the app writes to.
Remove 7 test files fully covered by e2e tests (admin, account, models,
API routes, integration MFA/OIDC, notifications). Trim 5 more files to
keep only edge cases not reachable via e2e.
Fix conftest to replace wiregui.db engine/session at import time so all
code uses the test database. Use session-scoped tables with per-test
savepoint isolation to prevent data leaking between tests.