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/Repositories/Viewer/IViewerRepository.cs

@@ -51,6 +51,14 @@ public interface IViewerRepository
     /// </summary>
     Task<Models.Viewer?> LoadForMatchContextAsync(long viewerId);
 
+    /// <summary>
+    /// Focused load for class-XP grants: viewer with owned <c>Classes</c> collection and
+    /// each <c>ViewerClassData.Class</c> nav ref included. Tracked (not AsNoTracking) so
+    /// the caller can mutate <c>Exp</c>/<c>Level</c> and <c>SaveChangesAsync</c>. Returns
+    /// null if the viewer does not exist.
+    /// </summary>
+    Task<Models.Viewer?> LoadForBattleXpGrantAsync(long viewerId, CancellationToken ct = default);
+
     /// <summary>Sets Viewer.GuildId to <paramref name="guildId"/>. No-op if the viewer does not exist.</summary>
     Task SetGuildIdAsync(long viewerId, int guildId, CancellationToken ct = default);
 

SVSim.Database/Repositories/Viewer/ViewerRepository.cs

@@ -273,6 +273,14 @@ public class ViewerRepository : IViewerRepository
             .FirstOrDefaultAsync(v => v.Id == viewerId);
     }
 
+    public Task<Models.Viewer?> LoadForBattleXpGrantAsync(long viewerId, CancellationToken ct = default)
+    {
+        return _dbContext.Set<Models.Viewer>()
+            .Include(v => v.Classes)
+                .ThenInclude(c => c.Class)
+            .FirstOrDefaultAsync(v => v.Id == viewerId, ct);
+    }
+
     public async Task SetGuildIdAsync(long viewerId, int guildId, CancellationToken ct = default)
     {
         var viewer = await _dbContext.Set<Models.Viewer>()