perf(purchases): select_related(platform) on list_purchases (#68)

Final-review follow-up: LinkedPurchase reads purchase.platform per row;
eager-load it alongside the games prefetch to remove the residual N+1.

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

games/views/purchase.py

@@ -125,7 +125,9 @@ def _render_purchase_row(purchase):
 
 @login_required
 def list_purchases(request: HttpRequest) -> HttpResponse:
-    purchases = Purchase.objects.prefetch_related("games", "games__platform")
+    purchases = Purchase.objects.select_related("platform").prefetch_related(
+        "games", "games__platform"
+    )
 
     filter_json = request.GET.get("filter", "")
     if filter_json: