Query the categories column for POI and near search

Query the categories column for POI and near search

Replace the place_classtype_* table lookups in PoiSearch and NearSearch with queries against the ltree categories column  (categories <@ 'osm.<class>.<type>'), backed by the GiST index. SQLite, which has no ltree, falls back to class/type equality. Because a merged placex row now carries several categories, both searches report the searched category rather than the row's class/type. get_class_table() is kept with a TODO; the place_classtype_* tables are removed in a follow-up PR.