Separated completeness and area_type fields

Factored out the proposal and conversation from https://git.gfz-potsdam.de/dynamicexposure/openbuildingmap/database-obmtiles/-/merge_requests/12#note_56310

\apporve @ds @nicgar @prehn

migrations/00006_separate_area_type_and_completeness.sql

+ALTER TABLE public.obm_tiles 
+    RENAME COLUMN completeness TO area_type;
+
+ALTER TABLE public.obm_tiles
+    ALTER COLUMN area_type [SET DATA] TYPE bit(4);
+
+ALTER TABLE public.obm_tiles 
+    ADD COLUMN IF NOT EXISTS completeness float;