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>

backend/models.py

@@ -133,6 +133,10 @@ class FactOfstedInspection(Base):
     leadership_management = Column(Integer)
     early_years_provision = Column(Integer)
     sixth_form_provision = Column(Integer)
+    # Ungraded (Section 8) inspection: raw outcome text and the grade parsed from
+    # it (fallback for schools with no graded overall effectiveness).
+    ungraded_outcome = Column(String(100))
+    ungraded_grade = Column(Integer)
     rc_safeguarding_met = Column(Boolean)
     rc_inclusion = Column(Integer)
     rc_curriculum_teaching = Column(Integer)