feat(pipeline): add legacy KS4 backfill (2015/16–2018/19)

Mirrors the existing legacy KS2 pattern to fill the gap before EES hosted
KS4 data. Four files changed:

- tap-uk-ees: LegacyKS4Stream downloads each year's DfE Compare School
  Performance ZIP, extracts england_ks4final.csv, maps 416 legacy columns
  to Singer fields, strips % suffixes. Registered in discover_streams().
  TapUKEES.config_jsonschema gains legacy_ks4_urls setting.

- stg_legacy_ks4.sql: safe_numeric casts + NULL placeholders for columns
  not present in legacy format (ebacc_avg_score, gcse_grade_91_pct,
  prior_attainment_avg, sen_pct).

- int_ks4_with_lineage.sql: adds all_ks4 CTE unioning stg_ees_ks4 and
  stg_legacy_ks4, matching the int_ks2_with_lineage pattern.

- _stg_sources.yml + meltano.yml: source declaration and setting definition
  for legacy_ks4. URLs configured per-year once provided.

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

pipeline/transform/models/intermediate/int_ks4_with_lineage.sql

@@ -1,6 +1,13 @@
 -- Intermediate model: KS4 data chained across academy conversions
+-- Unions EES (2023/24 onwards) and legacy (2015/16–2018/19) school-level data
 
-with current_ks4 as (
+with all_ks4 as (
+    select * from {{ ref('stg_ees_ks4') }}
+    union all
+    select * from {{ ref('stg_legacy_ks4') }}
+),
+
+current_ks4 as (
     select
         urn as current_urn,
         urn as source_urn,
@@ -11,8 +18,8 @@ with current_ks4 as (
         english_maths_strong_pass_pct, english_maths_standard_pass_pct,
         ebacc_entry_pct, ebacc_strong_pass_pct, ebacc_standard_pass_pct, ebacc_avg_score,
         gcse_grade_91_pct,
-        sen_pct, sen_ehcp_pct, sen_support_pct
-    from {{ ref('stg_ees_ks4') }}
+        sen_pct, sen_support_pct, sen_ehcp_pct
+    from all_ks4
 ),
 
 predecessor_ks4 as (
@@ -27,12 +34,12 @@ predecessor_ks4 as (
         ks4.english_maths_strong_pass_pct, ks4.english_maths_standard_pass_pct,
         ks4.ebacc_entry_pct, ks4.ebacc_strong_pass_pct, ks4.ebacc_standard_pass_pct, ks4.ebacc_avg_score,
         ks4.gcse_grade_91_pct,
-        ks4.sen_pct, ks4.sen_ehcp_pct, ks4.sen_support_pct
-    from {{ ref('stg_ees_ks4') }} ks4
+        ks4.sen_pct, ks4.sen_support_pct, ks4.sen_ehcp_pct
+    from all_ks4 ks4
     inner join {{ ref('int_school_lineage') }} lin
         on ks4.urn = lin.predecessor_urn
     where not exists (
-        select 1 from {{ ref('stg_ees_ks4') }} curr
+        select 1 from all_ks4 curr
         where curr.urn = lin.current_urn
           and curr.year = ks4.year
     )