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/meltano.yml

@@ -26,6 +26,9 @@ plugins:
         - name: legacy_ks2_urls
           kind: object
           description: "Year code → URL mapping for legacy KS2 CSVs"
+        - name: legacy_ks4_urls
+          kind: object
+          description: "Year code → URL mapping for legacy KS4 ZIPs (england_ks4final.csv inside)"
       config:
         legacy_ks2_urls:
           "201516": "http://10.0.1.224:8081/filebrowser/api/public/dl/R9jjXFWa?inline=true"