feat(replay): add BattleHistoryWriter with 50-row per-viewer retention

Idempotent on (ViewerId, BattleId); evicts oldest CreateTime row when
at cap. No-op when ctx is null (server-restart safety).

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

SVSim.Database/Services/Replay/IBattleHistoryWriter.cs

@@ -0,0 +1,16 @@
+namespace SVSim.Database.Services.Replay;
+
+/// <summary>
+/// Persists battle finishes to ViewerBattleHistory for the /replay/info list view.
+/// </summary>
+public interface IBattleHistoryWriter
+{
+    /// <summary>
+    /// Insert a history row for (viewerId, ctx.BattleId). No-op when ctx is null
+    /// (missing context = server restart mid-battle; warn-log and continue).
+    /// Idempotent on the composite PK — duplicate calls skip silently.
+    /// Enforces 50-row per-viewer retention by evicting the oldest CreateTime row
+    /// when at cap before insert.
+    /// </summary>
+    Task RecordAsync(long viewerId, BattleContext? ctx, bool isWin, CancellationToken ct);
+}