fix(dbt): compute geom from easting/northing in dim_location

Convert GIAS British National Grid coordinates (EPSG:27700) to WGS84
(EPSG:4326) directly in the dbt model. The geocode script backfills
schools missing easting/northing via Postcodes.io.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pipeline/transform/models/marts/dim_location.sql

@@ -1,5 +1,6 @@
 -- Mart: School location dimension — one row per URN, PostGIS-enabled
--- The geom column is populated by a post-hook or the geocode script.
+-- Geometry derived from GIAS easting/northing (British National Grid → WGS84).
+-- For schools missing easting/northing, the geocode script backfills via Postcodes.io.
 
 select
     s.urn,
@@ -13,7 +14,12 @@ select
     s.parliamentary_constituency,
     s.urban_rural,
     s.easting,
-    s.northing
+    s.northing,
+    case
+        when s.easting is not null and s.northing is not null
+        then ST_Transform(ST_SetSRID(ST_MakePoint(s.easting, s.northing), 27700), 4326)
+        else null
+    end as geom
 from {{ ref('stg_gias_establishments') }} s
 where s.status = 'Open'
   and s.postcode is not null