fix(api): accept academic-year codes in rankings year filter

The rankings endpoint validated year with le=2100, but the database
stores academic-year codes like 201819, so any explicit year selection
returned a 422 and the rankings page rendered its empty state. Widen
the bound to cover the codes and extend the e2e journey to pick a
specific year and assert the table stays populated.

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

backend/app.py

@@ -834,7 +834,10 @@ async def get_rankings(
     request: Request,
     metric: str = Query("rwm_expected_pct", description="Metric to rank by", max_length=50),
     year: Optional[int] = Query(
-        None, description="Specific year (defaults to most recent)", ge=2000, le=2100
+        None,
+        description="Academic year code, e.g. 201819 (defaults to most recent)",
+        ge=2000,
+        le=210100,
     ),
     limit: int = Query(20, ge=1, le=100, description="Number of schools to return"),
     local_authority: Optional[str] = Query(