fix(api): serialize schools that have no performance rows

Schools without KS2/KS4 results (special post-16 institutions, sixth-form
centres, PRUs, new schools) come back from the marts LEFT JOIN with NaN in
every numeric column. school_info passed those raw pandas values straight
into JSONResponse, which renders with allow_nan=False, so the detail
endpoint 500d and the frontend turned that into a 404 on every such SEO
landing page.

Run school_info values through convert_to_native (the same treatment
yearly_data already gets), add backend unit tests plus a pytest step in PR
checks, and an e2e journey that finds a results-less school via the search
API and asserts its page renders.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/app.py

@@ -33,7 +33,7 @@ from .data_loader import (
 )
 from .data_loader import get_data_info as get_db_info
 from .schemas import METRIC_DEFINITIONS, RANKING_COLUMNS, SCHOOL_COLUMNS
-from .utils import clean_for_json
+from .utils import clean_for_json, convert_to_native
 
 # Values to exclude from filter dropdowns (empty strings, non-applicable labels)
 EXCLUDED_FILTER_VALUES = {"", "Not applicable", "Does not apply"}
@@ -582,8 +582,13 @@ async def get_school_details(request: Request, urn: int):
     except Exception:
         pass
 
-    return {
-        "school_info": {
+    # Schools with no performance rows (post-16 institutions, PRUs, new
+    # schools) carry NaN in every LEFT-JOINed numeric column; NaN reaching
+    # JSONResponse raises ValueError, so school_info needs the same
+    # conversion yearly_data gets from clean_for_json.
+    school_info = {
+        k: convert_to_native(v)
+        for k, v in {
             "urn": urn,
             "school_name": latest.get("school_name", ""),
             "local_authority": latest.get("local_authority", ""),
@@ -601,7 +606,11 @@ async def get_school_details(request: Request, urn: int):
             "total_pupils": latest.get("gias_total_pupils"),
             "trust_name": latest.get("trust_name"),
             "gender": latest.get("gender"),
-        },
+        }.items()
+    }
+
+    return {
+        "school_info": school_info,
         "yearly_data": clean_for_json(school_data),
         # Supplementary data (null if not yet populated by Kestra)
         "ofsted": supplementary.get("ofsted"),