Phase 18: data expansion and market pricing fix

Four new card files (Serie A, Ligue 1, Primeira Liga, Eredivisie) add 45
cards across Italian, French, Portuguese, and Dutch football, bringing the
total card pool from 115 to 160. Each file uses fictional club/player
names and null image_path, keeping the project free of copyrighted assets.

Ten new SBC challenges (league_sbcs.json) targeting specific leagues and
nations increase the total from 7 to 17 challenges. Includes league purity
SBCs (Premier League, Bundesliga, Serie A, Ligue 1, Eredivisie), nation
combo SBCs (Iberian Derby, South American Fire), and utility SBCs (elite
strikers mini-submission, bronze-to-silver recycler, world tour).

NPC market refresh now shuffles the card pool before picking 24 listings
(was: take first 20 unshuffled), and price variance tightened from ±100%
to ±25% of the OVR-tier base price for more realistic NPC competition.

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

tests/integration_test.rs

@@ -1506,7 +1506,7 @@ async fn test_match_result_includes_level_ups_on_first_win() {
 
 #[tokio::test]
 async fn test_level_up_grants_milestone_pack_at_level_5() {
-    use openfut_core::models::profile::{level_for_xp, pack_for_level, XP_THRESHOLDS};
+    use openfut_core::models::profile::{level_for_xp, pack_for_level};
     // Level 5 requires 3000 XP total
     assert_eq!(level_for_xp(3000), 5);
     assert_eq!(pack_for_level(5), Some("bronze_pack"));