Fix postcode search ValueError when calculating distances

Use direct bracket indexing instead of .get() for pandas Series
row access in calc_distance function to ensure scalar values
are returned for pd.isna() checks.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app.py

@@ -277,7 +277,7 @@ async def get_schools(
 
             # Filter by distance using pre-geocoded lat/long from database
             def calc_distance(row):
-                if pd.isna(row.get("latitude")) or pd.isna(row.get("longitude")):
+                if pd.isna(row["latitude"]) or pd.isna(row["longitude"]):
                     return float("inf")
                 return haversine_distance(
                     search_coords[0],