feat: ingest official DfE KS2 national averages from EES data catalogue

Replaces computed means from our school dataset with the published DfE
national headline figures for the KS2 chart reference line.

- tap-uk-ees: new EESKs2NationalStream fetches the stable EES data-catalogue
  CSV (one row per year, England national total, AllSchools filter)
- dbt staging: stg_ees_ks2_national normalises columns, casts to float,
  filters to years >= 201617
- dbt mart: fact_ks2_national_averages — one row per year, official figures
- backend/models: Ks2NationalAverage SQLAlchemy model
- backend/app: /api/national-averages queries the mart for KS2 by_year;
  secondary by_year stays computed (no DfE KS4 national dataset yet)
- DAG: extract_ks2_national task added to school_data_annual_ees,
  runs in parallel with the main EES extract

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

pipeline/transform/models/marts/_marts_schema.yml

@@ -111,6 +111,12 @@ models:
       - name: urn
         tests: [not_null]
 
+  - name: fact_ks2_national_averages
+    description: Official DfE KS2 national headline averages — one row per academic year
+    columns:
+      - name: year
+        tests: [not_null, unique]
+
   - name: fact_deprivation
     description: IDACI deprivation index — one row per URN
     columns:

pipeline/transform/models/marts/fact_ks2_national_averages.sql

@@ -0,0 +1,25 @@
+{{ config(materialized='table') }}
+
+-- Mart: Official DfE KS2 national headline averages — one row per academic year.
+-- These are the published England-wide figures, not computed means from our school dataset.
+-- Used by the /api/national-averages endpoint to provide accurate per-year reference lines
+-- on the school history chart and for hero stat comparisons.
+
+select
+    year,
+    rwm_expected_pct,
+    rwm_high_pct,
+    reading_expected_pct,
+    reading_high_pct,
+    reading_avg_score,
+    writing_expected_pct,
+    writing_gd_pct,
+    maths_expected_pct,
+    maths_high_pct,
+    maths_avg_score,
+    gps_expected_pct,
+    gps_high_pct,
+    gps_avg_score,
+    science_expected_pct
+from {{ ref('stg_ees_ks2_national') }}
+order by year