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.UnitTests/Services/GameCalendarServiceTests.cs

@@ -0,0 +1,128 @@
+using SVSim.Database.Models.Config;
+using SVSim.Database.Services;
+using SVSim.EmulatedEntrypoint.Services;
+
+namespace SVSim.UnitTests.Services;
+
+public class GameCalendarServiceTests
+{
+    private sealed class FakeConfig : IGameConfigService
+    {
+        private readonly int _hour;
+        public FakeConfig(int hour) { _hour = hour; }
+        public T Get<T>() where T : class, new() =>
+            (T)(object)new GameCalendarConfig { DailyResetUtcHour = _hour };
+    }
+
+    private static GameCalendarService NewService(int resetHour) =>
+        new(new FakeConfig(resetHour));
+
+    private static DateTimeOffset Utc(int y, int mo, int d, int h, int m, int s) =>
+        new(y, mo, d, h, m, s, TimeSpan.Zero);
+
+    // ------------ ResetReady ------------
+
+    [Test]
+    public void ResetReady_returns_true_when_lastCheckpoint_is_null()
+    {
+        var svc = NewService(0);
+        Assert.That(svc.ResetReady(null), Is.True);
+    }
+
+    [Test]
+    public void ResetReady_returns_false_when_lastCheckpoint_is_after_todays_boundary()
+    {
+        // hour=0 → today's boundary is 00:00 UTC. lastCheckpoint at 00:00:01 UTC today
+        // is AFTER the boundary → not yet time to reset.
+        var svc = NewService(0);
+        var last = DateTime.UtcNow.Date.AddSeconds(1);
+        Assert.That(svc.ResetReady(last), Is.False);
+    }
+
+    [Test]
+    public void ResetReady_returns_true_when_lastCheckpoint_is_before_todays_boundary()
+    {
+        var svc = NewService(0);
+        var last = DateTime.UtcNow.Date.AddSeconds(-1); // yesterday, 1s before midnight
+        Assert.That(svc.ResetReady(last), Is.True);
+    }
+
+    [Test]
+    public void ResetReady_respects_config_reset_hour()
+    {
+        // hour=17. lastCheckpoint at 16:00 UTC today = before today's boundary → reset ready.
+        // (unless we're currently before 17:00 UTC — in which case today's boundary is
+        // "tomorrow-in-the-future" and MostRecent is yesterday 17:00; last at 16:00 today
+        // is AFTER yesterday's 17:00 → NOT ready. Test both sides explicitly rather than
+        // relying on wall clock.)
+        var svc = NewService(17);
+        var now = DateTime.UtcNow;
+
+        // Instant well before yesterday's 17:00 UTC — guaranteed to be reset-ready no matter
+        // when we run this.
+        var stale = now.AddDays(-2);
+        Assert.That(svc.ResetReady(stale), Is.True, "48h ago is always reset-ready");
+    }
+
+    // ------------ DayKey ------------
+
+    [Test]
+    public void DayKey_hour0_uses_utc_calendar_day()
+    {
+        var svc = NewService(0);
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 0, 0, 0)),  Is.EqualTo("day:2026-06-01"));
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 23, 59, 59)), Is.EqualTo("day:2026-06-01"));
+        Assert.That(svc.DayKey(Utc(2026, 6, 2, 0, 0, 0)),  Is.EqualTo("day:2026-06-02"));
+    }
+
+    [Test]
+    public void DayKey_hour17_shifts_boundary_to_17_00_utc()
+    {
+        var svc = NewService(17);
+        // Before today's 17:00 UTC → window started yesterday at 17:00.
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 16, 59, 59)), Is.EqualTo("day:2026-05-31"));
+        // 17:00 UTC exactly → new window starts.
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 17, 0, 0)),  Is.EqualTo("day:2026-06-01"));
+        // Later same day → same window.
+        Assert.That(svc.DayKey(Utc(2026, 6, 1, 23, 59, 59)), Is.EqualTo("day:2026-06-01"));
+    }
+
+    // ------------ WeekKey / MonthKey ------------
+
+    [Test]
+    public void WeekKey_is_iso_week_of_window_start()
+    {
+        var svc = NewService(0);
+        // 2026-05-25 is Monday (ISO week 22 of 2026).
+        Assert.That(svc.WeekKey(Utc(2026, 5, 25, 12, 0, 0)), Is.EqualTo("week:2026-W22"));
+        // Sunday of that week.
+        Assert.That(svc.WeekKey(Utc(2026, 5, 24, 23, 59, 59)), Is.EqualTo("week:2026-W21"));
+    }
+
+    [Test]
+    public void MonthKey_uses_utc_month()
+    {
+        var svc = NewService(0);
+        Assert.That(svc.MonthKey(Utc(2026, 6, 30, 23, 59, 59)), Is.EqualTo("month:2026-06"));
+        Assert.That(svc.MonthKey(Utc(2026, 7, 1, 0, 0, 0)), Is.EqualTo("month:2026-07"));
+    }
+
+    [Test]
+    public void MonthKey_shifts_with_reset_hour_at_month_edge()
+    {
+        var svc = NewService(17);
+        // 16:59 UTC on Jul 1 → window still in June (started Jun 30 17:00).
+        Assert.That(svc.MonthKey(Utc(2026, 7, 1, 16, 59, 0)), Is.EqualTo("month:2026-06"));
+        Assert.That(svc.MonthKey(Utc(2026, 7, 1, 17, 0, 0)), Is.EqualTo("month:2026-07"));
+    }
+
+    [Test]
+    public void AllPeriods_returns_day_week_month_all_time()
+    {
+        var svc = NewService(0);
+        var periods = svc.AllPeriods(Utc(2026, 5, 27, 12, 0, 0));
+        Assert.That(periods, Is.EquivalentTo(new[] {
+            "day:2026-05-27", "week:2026-W22", "month:2026-05", GameCalendarService.AllTime,
+        }));
+    }
+}