feat(battle-xp): config section + viewer loader for class XP grants

Adds BattleXpConfig ([ConfigSection("BattleXp")]) with global
XpPerWin/XpPerLoss plus per-mode nullable overrides. Adds BattleXpMode
enum and IViewerRepository.LoadForBattleXpGrantAsync — focused tracked
load with viewer.Classes + Class nav ref included, for the service
about to be introduced.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.Database/Services/BattleXp/BattleXpMode.cs

@@ -0,0 +1,11 @@
+namespace SVSim.Database.Services.BattleXp;
+
+public enum BattleXpMode
+{
+    Practice,
+    Rank,
+    Free,
+    ArenaTwoPick,
+    Colosseum,
+    Story,
+}