The PRECON_INDEX list was imported by value in routes/shared.py.
When download_precon_database() or load_precon_index() reassigned it
with 'PRECON_INDEX = ...', the new list object was only visible in
precon_service module scope — shared.PRECON_INDEX (and thus the
/api/lab/precons endpoint) still pointed to the original empty [].
Fix: use PRECON_INDEX.clear() + .extend() instead of reassignment
so all modules sharing the same list reference see the updated data.