feat(search): phases in the order a child meets them

The phase filter listed GIAS phases alphabetically, so "16 plus" and
"All-through" came before Nursery. /api/filters (and the result-scoped
list) now order them Nursery, Primary, Middle deemed primary, Middle
deemed secondary, Secondary, 16 plus, then All-through, which spans the
whole path. A phase GIAS adds later follows the known ones, A-Z.

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

backend/schemas.py

@@ -544,6 +544,19 @@ PHASE_GROUPS: dict[str, set[str]] = {
     "all-through": {"all-through"},
 }
 
+# GIAS phases in the order a child meets them, for the phase filter's options.
+# All-through spans the whole path, so it follows the stages. Lowercased, as
+# PHASE_GROUPS is, so a change of case in the GIAS label keeps its place.
+PHASE_ORDER: list[str] = [
+    "nursery",
+    "primary",
+    "middle deemed primary",
+    "middle deemed secondary",
+    "secondary",
+    "16 plus",
+    "all-through",
+]
+
 # School listing columns
 SCHOOL_COLUMNS = [
     "urn",