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/tests/test_phase_filter.py

@@ -82,3 +82,25 @@ def test_grouped_phases_still_take_in_their_related_phases(client):
 
 def test_an_unknown_phase_returns_nothing_rather_than_everything(client):
     assert _urns(client, "kindergarten") == []
+
+
+def test_filters_lists_phases_in_the_order_a_child_meets_them(client):
+    # Alphabetical put "16 plus" and "All-through" first and Nursery fifth.
+    # All-through spans the whole path, so it comes after the stages.
+    assert client.get("/api/filters").json()["phases"] == [
+        "Nursery",
+        "Primary",
+        "Middle deemed primary",
+        "Middle deemed secondary",
+        "Secondary",
+        "16 plus",
+        "All-through",
+    ]
+
+
+def test_an_unknown_phase_follows_the_known_ones():
+    from backend.app import order_phases
+
+    assert order_phases(["Secondary", "Zeta", "Alpha", "Nursery"]) == [
+        "Nursery", "Secondary", "Alpha", "Zeta",
+    ]