fix(api): treat "16 plus" as secondary, the way PHASE_GROUPS already does

GIAS phase 6 is "16 plus", and PHASE_GROUPS deliberately files it in the
secondary group. The payload helper decided the same question with
`"secondary" in phase_text`, which that value does not satisfy — so a
sixth-form college was handed the primary bucket and offered infant schools
as its peers, with the KS2 metric key to label them. No crash; just a page
confidently showing the wrong schools.

The decision now lives in similar_schools.is_secondary_phase, beside the
PHASE_GROUPS bucket it selects from, so the two cannot drift again. A test
pins them together.

The same binary assumption had a second output. computeSchoolFlags tests for
the substring too, so a 16-plus school renders the primary template, and the
composer was labelling the section from the template: "Other primary schools
near <sixth form college>" above a row of secondaries. The section now
derives its noun from the school's own phase, which also removes the
duplicated wording from both composers. A 16-plus school's candidates span
the whole secondary group, so no single noun fits and it gets the honest
general one.

Reported in review on #150.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/similar_schools.py

@@ -89,6 +89,24 @@ def phase_label(phase: str | None) -> str:
     return f"{text.capitalize()} school"
 
 
+def is_secondary_phase(phase: str | None) -> bool:
+    """Whether this phase takes the secondary side: secondary group membership,
+    minus all-through.
+
+    Membership is read from PHASE_GROUPS rather than tested with `"secondary" in
+    phase`, because that substring misses "16 plus" — GIAS phase 6, which
+    PHASE_GROUPS deliberately files as secondary. The substring version fails
+    silently rather than loudly: a sixth-form college is simply handed the
+    primary bucket and offered infant schools as peers.
+
+    All-through is the exception. PHASE_GROUPS lists it on both sides because it
+    belongs on both phases' place pages, but the detail page renders it with the
+    primary template, and the metric follows the template.
+    """
+    text = (phase or "").strip().lower()
+    return text != "all-through" and text in PHASE_GROUPS["secondary"]
+
+
 def _phase_group(is_secondary: bool) -> set[str]:
     return PHASE_GROUPS["secondary" if is_secondary else "primary"]