fix: ensure count is never null in GET /refractor/cards (#183)

Guards against Peewee 3.17.9 returning None from .count() on a
complex multi-join query when 0 rows match the filter set.

Closes #183

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

app/routers_v2/refractor.py

@@ -211,7 +211,7 @@ async def list_card_states(
     if evaluated_only:
         query = query.where(RefractorCardState.last_evaluated_at.is_null(False))
 
-    total = query.count()
+    total = query.count() or 0
     items = []
     for state in query.offset(offset).limit(limit):
         player_name = None