feat(api): name each school's type group, and send search its nursery provision

The school page and the search rows now print a school's type in the search
filter's terms ("State school", "Independent school") instead of GIAS's 34
establishment types. The list, place and detail payloads carry type_group,
computed with the filter's own type_group_for, so the two never disagree; a
type in no group stays null and the page prints the register's name.

Search rows flag a nursery class, so nursery_provision joins SCHOOL_COLUMNS.

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

backend/schemas.py

@@ -569,6 +569,7 @@ SCHOOL_COLUMNS = [
     "religious_denomination",
     "age_range",
     "has_sixth_form",
+    "nursery_provision",
     "status",
     "gender",
     "admissions_policy",