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/BattleHistoryWriter.cs

@@ -0,0 +1,73 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using SVSim.Database.Models;
+
+namespace SVSim.Database.Services.Replay;
+
+public sealed class BattleHistoryWriter : IBattleHistoryWriter
+{
+    internal const int RetentionCap = 50;
+
+    private readonly SVSimDbContext _db;
+    private readonly ILogger<BattleHistoryWriter> _log;
+
+    public BattleHistoryWriter(SVSimDbContext db, ILogger<BattleHistoryWriter> log)
+    {
+        _db = db;
+        _log = log;
+    }
+
+    public async Task RecordAsync(long viewerId, BattleContext? ctx, bool isWin, CancellationToken ct)
+    {
+        if (ctx is null)
+        {
+            _log.LogWarning(
+                "BattleHistoryWriter.RecordAsync called with null context for viewer {ViewerId} - " +
+                "likely missed start-time Set (server restart or non-tracked family). Skipping.",
+                viewerId);
+            return;
+        }
+
+        var existing = await _db.ViewerBattleHistories
+            .AnyAsync(h => h.ViewerId == viewerId && h.BattleId == ctx.BattleId, ct);
+        if (existing) return; // idempotent
+
+        var count = await _db.ViewerBattleHistories
+            .CountAsync(h => h.ViewerId == viewerId, ct);
+        if (count >= RetentionCap)
+        {
+            var oldest = await _db.ViewerBattleHistories
+                .Where(h => h.ViewerId == viewerId)
+                .OrderBy(h => h.CreateTime)
+                .FirstAsync(ct);
+            _db.ViewerBattleHistories.Remove(oldest);
+        }
+
+        _db.ViewerBattleHistories.Add(new ViewerBattleHistory
+        {
+            ViewerId            = viewerId,
+            BattleId            = ctx.BattleId,
+            BattleType          = ctx.BattleType,
+            DeckFormat          = ctx.DeckFormat,
+            TwoPickType         = ctx.TwoPickType,
+            IsLimitTurn         = 0,
+            SelfClassId         = ctx.SelfClassId,
+            SelfSubClassId      = ctx.SelfSubClassId,
+            SelfCharaId         = ctx.SelfCharaId,
+            SelfRotationId      = ctx.SelfRotationId,
+            OpponentClassId     = ctx.OpponentClassId,
+            OpponentSubClassId  = ctx.OpponentSubClassId,
+            OpponentCharaId     = ctx.OpponentCharaId,
+            OpponentName        = ctx.OpponentName,
+            OpponentCountryCode = ctx.OpponentCountryCode,
+            OpponentEmblemId    = ctx.OpponentEmblemId,
+            OpponentDegreeId    = ctx.OpponentDegreeId,
+            OpponentRotationId  = ctx.OpponentRotationId,
+            IsWin               = isWin,
+            BattleStartTime     = ctx.BattleStartTime,
+            CreateTime          = DateTime.UtcNow,
+        });
+
+        await _db.SaveChangesAsync(ct);
+    }
+}

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);
+}