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.UnitTests/Services/BattleHistoryWriterTests.cs

@@ -0,0 +1,95 @@
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.DependencyInjection;
+using SVSim.Database;
+using SVSim.Database.Services.Replay;
+using SVSim.UnitTests.Infrastructure;
+
+namespace SVSim.UnitTests.Services;
+
+public class BattleHistoryWriterTests
+{
+    private static BattleContext MakeCtx(long battleId) => new(
+        BattleId: battleId,
+        BattleType: 2, DeckFormat: 0, TwoPickType: 0,
+        SelfClassId: 1, SelfSubClassId: 0, SelfCharaId: 1, SelfRotationId: "0",
+        OpponentViewerId: 0, OpponentName: "Bot", OpponentClassId: 2,
+        OpponentSubClassId: 0, OpponentCharaId: 1, OpponentCountryCode: "",
+        OpponentEmblemId: 0, OpponentDegreeId: 0, OpponentRotationId: "0",
+        BattleStartTime: DateTime.UtcNow);
+
+    [Test]
+    public async Task RecordAsync_writes_row()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(steamId: 76_561_198_000_000_001UL);
+
+        using var scope = factory.Services.CreateScope();
+        var writer = scope.ServiceProvider.GetRequiredService<IBattleHistoryWriter>();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        await writer.RecordAsync(viewerId, MakeCtx(battleId: 42), isWin: true, default);
+
+        var row = await db.ViewerBattleHistories
+            .SingleAsync(h => h.ViewerId == viewerId && h.BattleId == 42);
+        Assert.That(row.IsWin, Is.True);
+        Assert.That(row.OpponentName, Is.EqualTo("Bot"));
+    }
+
+    [Test]
+    public async Task RecordAsync_null_ctx_is_noop()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(steamId: 76_561_198_000_000_001UL);
+
+        using var scope = factory.Services.CreateScope();
+        var writer = scope.ServiceProvider.GetRequiredService<IBattleHistoryWriter>();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        await writer.RecordAsync(viewerId, ctx: null, isWin: true, default);
+
+        Assert.That(await db.ViewerBattleHistories.CountAsync(), Is.EqualTo(0));
+    }
+
+    [Test]
+    public async Task RecordAsync_duplicate_battle_id_skips_silently()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(steamId: 76_561_198_000_000_001UL);
+
+        using var scope = factory.Services.CreateScope();
+        var writer = scope.ServiceProvider.GetRequiredService<IBattleHistoryWriter>();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        await writer.RecordAsync(viewerId, MakeCtx(battleId: 42), isWin: true, default);
+        await writer.RecordAsync(viewerId, MakeCtx(battleId: 42), isWin: false, default);
+
+        // Original row is preserved; the second call is a no-op.
+        var row = await db.ViewerBattleHistories
+            .SingleAsync(h => h.ViewerId == viewerId && h.BattleId == 42);
+        Assert.That(row.IsWin, Is.True);
+    }
+
+    [Test]
+    public async Task RecordAsync_evicts_oldest_when_over_retention_cap()
+    {
+        using var factory = new SVSimTestFactory();
+        long viewerId = await factory.SeedViewerAsync(steamId: 76_561_198_000_000_001UL);
+
+        using var scope = factory.Services.CreateScope();
+        var writer = scope.ServiceProvider.GetRequiredService<IBattleHistoryWriter>();
+        var db = scope.ServiceProvider.GetRequiredService<SVSimDbContext>();
+
+        // Write 51 rows. The first (oldest CreateTime) should be evicted on row 51's insert.
+        for (long i = 1; i <= 51; i++)
+        {
+            await writer.RecordAsync(viewerId, MakeCtx(battleId: i), isWin: false, default);
+        }
+
+        var count = await db.ViewerBattleHistories.CountAsync(h => h.ViewerId == viewerId);
+        Assert.That(count, Is.EqualTo(50));
+
+        var hasOldest = await db.ViewerBattleHistories
+            .AnyAsync(h => h.ViewerId == viewerId && h.BattleId == 1);
+        Assert.That(hasOldest, Is.False, "battle_id=1 should have been evicted as the oldest");
+    }
+}