Fix collection stats query

Only include latest prices so the card count and value totals are
reported correctly.

tutor/database.py

@@ -373,6 +373,8 @@ async def collection_stats(db: psycopg.Cursor) -> dict:
                 FROM "copies"
                 JOIN "cards" USING ("scryfall_id")
                 LEFT JOIN "card_prices" USING ("scryfall_id")
+               WHERE "card_prices"."date" =
+                     (SELECT "value" FROM "vars" where "key" = 'last_update')
                 """
     )
     return await db.fetchone()