fix(search): fix load-more silently failing due to missing page_size param

Load-more requests read URL params (postcode, radius, etc.) but page_size
is never in the URL — it's hardcoded in page.tsx. Without it the backend
received page_size=None, hit a TypeError on (page-1)*None, returned 500,
and the silent catch left the user stuck on page 1.

In a dense area (e.g. Wimbledon SW19) 50 schools fit within ~1.8 miles,
so page 1 never shows anything beyond that regardless of selected radius.

Fix:
- Backend: give page_size a safe default of 25 instead of None
- Frontend: explicitly pass initialSchools.page_size in load-more params

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app.py

@@ -295,7 +295,7 @@ async def get_schools(
     postcode: Optional[str] = Query(None, description="Search near postcode", max_length=10),
     radius: float = Query(5.0, ge=0.1, le=50, description="Search radius in miles"),
     page: int = Query(1, ge=1, le=1000, description="Page number"),
-    page_size: int = Query(None, ge=1, le=100, description="Results per page"),
+    page_size: int = Query(25, ge=1, le=100, description="Results per page"),
     gender: Optional[str] = Query(None, description="Filter by gender (Mixed/Boys/Girls)", max_length=50),
     admissions_policy: Optional[str] = Query(None, description="Filter by admissions policy", max_length=100),
     has_sixth_form: Optional[str] = Query(None, description="Filter by sixth form presence: yes/no", max_length=3),