feat(pipeline): add Meltano + dbt + Airflow ELT pipeline scaffold

Replaces the hand-rolled integrator with a production-grade ELT pipeline
using Meltano (Singer taps), dbt Core (medallion architecture), and
Apache Airflow (orchestration). Adds Typesense for search and PostGIS
for geospatial queries.

- 6 custom Singer taps (GIAS, EES, Ofsted, Parent View, FBIT, IDACI)
- dbt project: 12 staging, 5 intermediate, 12 mart models
- 3 Airflow DAGs (daily/monthly/annual schedules)
- Typesense sync + batch geocoding scripts
- docker-compose: add Airflow, Typesense; upgrade to PostGIS
- Portainer stack definition matching live deployment topology

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

pipeline/transform/tests/assert_no_orphaned_facts.sql

@@ -0,0 +1,7 @@
+-- Custom test: All fact table URNs should exist in dim_school
+
+select f.urn
+from {{ ref('fact_ks2_performance') }} f
+left join {{ ref('dim_school') }} d on f.urn = d.urn
+where d.urn is null
+limit 10

pipeline/transform/tests/assert_urns_in_uk_bounds.sql

@@ -0,0 +1,13 @@
+-- Custom test: All geocoded schools should have coordinates within the UK
+
+select
+    urn,
+    easting,
+    northing
+from {{ ref('dim_location') }}
+where easting is not null
+  and northing is not null
+  and (
+    easting < 0 or easting > 700000
+    or northing < 0 or northing > 1300000
+  )