feat(ofsted): fall back to ungraded inspection outcome for school grade

Ungraded (Section 8) inspections don't assign a fresh grade — the export
only gives free text like "School remains Good". Parse that text into a
grade (remains Outstanding -> 1, remains Good -> 2, else null) and use it
as a last-resort fallback when no graded overall effectiveness exists.

Also retain schools that have only an ungraded inspection (no graded date)
by coalescing the inspection date, so ~8.5k previously-dropped schools now
carry a grade.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

pipeline/transform/models/marts/dim_school.sql

@@ -60,7 +60,9 @@ select
 
     -- Latest Ofsted (populated after monthly Ofsted pipeline runs)
     {% if ofsted_relation is not none %}
-    o.overall_effectiveness as ofsted_grade,
+    -- Prefer the graded overall effectiveness; fall back to the grade parsed
+    -- from the latest ungraded (Section 8) outcome when no graded grade exists.
+    coalesce(o.overall_effectiveness, o.ungraded_grade) as ofsted_grade,
     o.inspection_date as ofsted_date,
     o.framework as ofsted_framework
     {% else %}