perf(api): persist KS4 national averages as a mart; stop per-request aggregation

fact_ks4_national_averages is computed once at dbt build time (covered by
the EES DAG's stg_ees_ks4+ selector). _national_averages_payload now reads
both national-averages marts instead of scanning the performance dataframe
per year on every /api/compare request (~250ms saved per call). Fallback
for the deploy-before-DAG window computes the latest year only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0146VHeLAWjDVE2B5uU67jCB

pipeline/transform/models/marts/_marts_schema.yml

@@ -160,6 +160,12 @@ models:
       - name: year
         tests: [not_null, unique]
 
+  - name: fact_ks4_national_averages
+    description: Computed national KS4 averages (means across state schools in our dataset — not official DfE figures) — 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_ks4_national_averages.sql

@@ -0,0 +1,25 @@
+{{ config(materialized='table') }}
+
+-- Mart: Computed national KS4 averages — one row per academic year.
+-- Unlike fact_ks2_national_averages (official DfE figures), DfE publishes no
+-- KS4 national-headline dataset we ingest yet, so these are means computed
+-- across the state schools in our dataset. Computed once at build time so the
+-- API never has to aggregate the full performance table per request.
+-- Semantics match the API's previous per-request computation: rows where
+-- attainment_8_score is non-null; per-column means ignore NULLs.
+
+select
+    year,
+    round(avg(attainment_8_score)::numeric, 2)               as attainment_8_score,
+    round(avg(progress_8_score)::numeric, 2)                 as progress_8_score,
+    round(avg(english_maths_standard_pass_pct)::numeric, 2)  as english_maths_standard_pass_pct,
+    round(avg(english_maths_strong_pass_pct)::numeric, 2)    as english_maths_strong_pass_pct,
+    round(avg(ebacc_entry_pct)::numeric, 2)                  as ebacc_entry_pct,
+    round(avg(ebacc_standard_pass_pct)::numeric, 2)          as ebacc_standard_pass_pct,
+    round(avg(ebacc_strong_pass_pct)::numeric, 2)            as ebacc_strong_pass_pct,
+    round(avg(ebacc_avg_score)::numeric, 2)                  as ebacc_avg_score,
+    round(avg(gcse_grade_91_pct)::numeric, 2)                as gcse_grade_91_pct
+from {{ ref('fact_ks4_performance') }}
+where attainment_8_score is not null
+group by year
+order by year