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.