Parse ``LDR_TEST_MODE`` as a proper boolean when deciding whether to relax the SQLCipher KDF floor. Previously ``_get_min_kdf_iterations()`` used a bare truthiness check, so any non-empty value — including ``LDR_TEST_MODE=0`` or ``=false`` — enabled the relaxed test minimum, silently weakening database encryption for anyone setting the flag explicitly to disable it. ``PYTEST_CURRENT_TEST`` stays presence-based (pytest sets it to a non-boolean string).
