fix(backend): serve the current Ofsted status from fact_ofsted_latest

The list query and the batch Ofsted fetch each picked 'the latest row' of fact_ofsted_inspection themselves, and _ofsted_block carried an older ungraded grade forward when the latest graded inspection gave none. All three now read marts.fact_ofsted_latest. List rows: ofsted_grade is the grade still in force, ofsted_grade_date when it was awarded or confirmed, ofsted_date the latest visit. The ofsted block gains current_grade and latest_visit and loses grade_source; overall_effectiveness is the graded inspection's own result. A school with only an inspection stays publishable in the sitemap.

Requires fact_ofsted_latest (PR #183's pipeline run) on the database.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

backend/schemas.py

@@ -574,6 +574,7 @@ SCHOOL_COLUMNS = [
     "gender",
     "admissions_policy",
     "ofsted_grade",
+    "ofsted_grade_date",
     "ofsted_date",
     "ofsted_framework",
     "ofsted_rc_date",