feat(calendar): add IGameCalendarService — UTC reset-boundary primitive

New config-driven service that replaces JstPeriod. All timestamps are UTC;
one config value (GameCalendarConfig.DailyResetUtcHour, default 0 = midnight
UTC) drives both ResetReady checks and DayKey/WeekKey/MonthKey bucket keys.
Because both derive from the same MostRecentBoundary helper, they can never
disagree.

Prod parity is a config flip: set DailyResetUtcHour=17 in GameConfigs to get
the 02:00 JST boundary. No JST-flavored math anywhere in the code.

JstPeriod callsite migration + deletion follow in subsequent commits.

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

SVSim.EmulatedEntrypoint/Program.cs

@@ -127,6 +127,7 @@ public class Program
         builder.Services.AddScoped<IMatchContextBuilder, MatchContextBuilder>();
         builder.Services.AddScoped<IStoryService, StoryService>();
         builder.Services.AddScoped<ILoginBonusService, LoginBonusService>();
+        builder.Services.AddScoped<IGameCalendarService, GameCalendarService>();
         builder.Services.AddScoped<IDeckListBuilder, DeckListBuilder>();
         builder.Services.AddSingleton<IRandom, SystemRandom>();
         builder.Services.AddSingleton<PuzzleMissionEvaluator>();

SVSim.EmulatedEntrypoint/Services/GameCalendarService.cs

@@ -0,0 +1,88 @@
+using System.Globalization;
+using SVSim.Database.Models.Config;
+using SVSim.Database.Services;
+
+namespace SVSim.EmulatedEntrypoint.Services;
+
+/// <summary>
+/// Reset-boundary primitive. All timestamps are UTC. The boundary hour is a config
+/// value (<see cref="GameCalendarConfig.DailyResetUtcHour"/>); everything else — daily
+/// reset checks, bucket keys for <c>ViewerEventCounters</c> — derives from it.
+/// Because bucket keys and <see cref="ResetReady"/> share the same
+/// <see cref="MostRecentBoundary"/> math, they can never disagree.
+/// </summary>
+public interface IGameCalendarService
+{
+    /// <summary>
+    /// True if a reset boundary has passed since <paramref name="lastCheckpointUtc"/>.
+    /// Null → true (never checked in).
+    /// </summary>
+    bool ResetReady(DateTime? lastCheckpointUtc);
+
+    /// <summary>Bucket key for the current daily reset window: <c>"day:yyyy-MM-dd"</c>.</summary>
+    string DayKey(DateTimeOffset when);
+
+    /// <summary>Bucket key for the ISO week containing the current daily window's start.</summary>
+    string WeekKey(DateTimeOffset when);
+
+    /// <summary>Bucket key for the calendar month containing the current daily window's start.</summary>
+    string MonthKey(DateTimeOffset when);
+
+    /// <summary>Returns [day, week, month, all-time] for the given instant.</summary>
+    IReadOnlyList<string> AllPeriods(DateTimeOffset when);
+}
+
+public class GameCalendarService : IGameCalendarService
+{
+    public const string AllTime = "all-time";
+
+    private readonly IGameConfigService _config;
+
+    public GameCalendarService(IGameConfigService config)
+    {
+        _config = config;
+    }
+
+    public bool ResetReady(DateTime? lastCheckpointUtc)
+    {
+        if (lastCheckpointUtc is null) return true;
+        var now = DateTime.UtcNow;
+        var boundary = MostRecentBoundary(now);
+        return DateTime.SpecifyKind(lastCheckpointUtc.Value, DateTimeKind.Utc) < boundary;
+    }
+
+    public string DayKey(DateTimeOffset when)
+    {
+        var d = WindowStartDate(when);
+        return $"day:{d:yyyy-MM-dd}";
+    }
+
+    public string WeekKey(DateTimeOffset when)
+    {
+        var d = WindowStartDate(when);
+        var iso = ISOWeek.GetWeekOfYear(d);
+        var year = ISOWeek.GetYear(d);
+        return $"week:{year:D4}-W{iso:D2}";
+    }
+
+    public string MonthKey(DateTimeOffset when)
+    {
+        var d = WindowStartDate(when);
+        return $"month:{d:yyyy-MM}";
+    }
+
+    public IReadOnlyList<string> AllPeriods(DateTimeOffset when) => new[]
+    {
+        DayKey(when), WeekKey(when), MonthKey(when), AllTime,
+    };
+
+    private DateTime MostRecentBoundary(DateTime nowUtc)
+    {
+        int hour = _config.Get<GameCalendarConfig>().DailyResetUtcHour;
+        var todayBoundary = nowUtc.Date.AddHours(hour);
+        return nowUtc >= todayBoundary ? todayBoundary : todayBoundary.AddDays(-1);
+    }
+
+    private DateTime WindowStartDate(DateTimeOffset when) =>
+        MostRecentBoundary(when.UtcDateTime);
+}