fix(dbt): stg_gias_ukprn reads URN as the bigint target-postgres loads (H4)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

pipeline/transform/models/staging/stg_gias_ukprn.sql

@@ -8,4 +8,5 @@ select
     cast("URN" as integer)          as urn,
     nullif(trim("UKPRN"), '')       as ukprn
 from {{ source('raw', 'gias_establishments') }}
-where "URN" ~ '^[0-9]+$'
+-- target-postgres types URN as bigint (the tap declares an integer).
+where "URN" is not null