``vertical`` is one of: ``bap`` (Torget / used goods), ``homes`` (real
estate for sale), ``lettings`` (real estate for rent), ``cars_used``,
``cars_new`` (not yet supported by the scraper backend), ``jobs``.

``query`` is a free-text keyword. ``filters`` is a dict of extra
URL-query parameters understood by finn.no (e.g. ``{"price_to":"300000"}``
for cars, ``{"location":"1.20001.20061"}`` for Oslo).

Returns up to ~50 ``SearchResult`` entries. Fewer if finn.no returned
fewer matches; an empty list is a valid result.

If ``vertical`` is omitted the server tries a cache hint first, then
probes each vertical until one returns a valid page. Results are cached
for 24 hours in a local SQLite database.

Updates the saved search's ``last_checked_at`` and ``last_finnkodes``.
On the first call all current results are "new".