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.Database/Models/Config/GameCalendarConfig.cs

@@ -0,0 +1,18 @@
+namespace SVSim.Database.Models.Config;
+
+/// <summary>
+/// Reset boundaries for time-windowed game state (daily missions, daily-single packs,
+/// login bonuses). All timestamps in the codebase are UTC; this config only shifts
+/// where the day boundary falls.
+/// </summary>
+[ConfigSection("GameCalendar")]
+public class GameCalendarConfig
+{
+    /// <summary>
+    /// UTC hour (0-23) at which the daily reset occurs. Default 0 = midnight UTC.
+    /// Prod parity: 17 (= 02:00 JST). Changing the value at runtime is safe for the
+    /// ResetReady check but retroactively misaligns any DB rows keyed by DayKey /
+    /// WeekKey / MonthKey — treat as a startup constant in production.
+    /// </summary>
+    public int DailyResetUtcHour { get; set; } = 0;
+}