feat(api): serve similar schools on the detail endpoint

Rides in the existing payload rather than a new endpoint: the page already
makes one server fetch for its data, and /school/[slug] regenerates weekly,
so the per-request cost is paid once per school per week.

Wrapped so a failure in selection never 500s a page that is otherwise
complete — the posture get_supplementary_data already takes.

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

backend/app.py

@@ -41,6 +41,7 @@ from .data_loader import get_data_info as get_db_info
 from . import flags
 from .places import build_place_index, build_place_registry, places_for_urn
 from .schemas import METRIC_DEFINITIONS, PHASE_GROUPS, RANKING_COLUMNS, SCHOOL_COLUMNS
+from .similar_schools import select_similar
 from .utils import clean_for_json, convert_to_native
 
 # Values to exclude from filter dropdowns (empty strings, non-applicable labels)
@@ -265,6 +266,28 @@ def _places_payload(urn: int) -> list[dict]:
     return payload
 
 
+def _similar_schools_payload(urn: int, phase: str | None) -> list[dict]:
+    """Nearby schools this page may offer as alternatives.
+
+    Wrapped: a failure in selection must never 500 a page that is otherwise
+    complete, which is the posture get_supplementary_data already takes. The
+    section simply does not render.
+    """
+    try:
+        phase_text = (phase or "").lower()
+        # All-through schools render with the primary template, which is what
+        # decides the metric, so they are not secondary here.
+        is_secondary = phase_text != "all-through" and "secondary" in phase_text
+        return select_similar(load_latest_school_data(), int(urn), is_secondary)
+    except Exception:
+        import logging
+
+        logging.getLogger(__name__).exception(
+            "Similar schools selection failed for urn=%s", urn
+        )
+        return []
+
+
 def _place_sitemap_rows(kinds: tuple[str, ...], registry=None) -> list[str]:
     """A <url> per place, plus a phase variant wherever that phase clears the
     threshold on its own.
@@ -975,6 +998,11 @@ async def get_school_details(request: Request, urn: int):
         # and authority both fall below the publish threshold has nowhere to
         # point, and the page renders without the module.
         "places": _places_payload(urn),
+        # Nearby schools of the same phase and a comparable intake. Always
+        # present on a build with this code; the frontend treats absent and
+        # empty identically, which is what lets the two images deploy
+        # independently.
+        "similar_schools": _similar_schools_payload(urn, latest.get("phase")),
         "yearly_data": clean_for_json(school_data),
         # Supplementary data (null if not yet populated by Kestra)
         "ofsted": supplementary.get("ofsted"),