feat: add campaign templates and recurring schedules

src/GmRelay.Web/Services/BatchSchedulePlanner.cs

@@ -18,8 +18,33 @@ public sealed record WebSessionBatch(
     int SessionCount,
     string NotificationMode = SessionNotificationModeExtensions.GroupAndDirectValue);
 
+public sealed record WebCampaignTemplate(
+    Guid Id,
+    Guid GroupId,
+    string Name,
+    string Title,
+    string JoinLink,
+    int SessionCount,
+    int IntervalDays,
+    int? MaxPlayers,
+    string NotificationMode,
+    DateTime CreatedAt,
+    DateTime UpdatedAt);
+
+public sealed record CreateCampaignTemplateRequest(
+    string Name,
+    string Title,
+    string JoinLink,
+    int SessionCount,
+    int IntervalDays,
+    int? MaxPlayers,
+    SessionNotificationMode NotificationMode);
+
 public static class BatchSchedulePlanner
 {
+    private const int MaxTemplateSessionCount = 52;
+    private const int MaxTemplateIntervalDays = 365;
+
     public static IReadOnlyList<DateTime> BuildFixedIntervalSchedule(
         IEnumerable<DateTime> currentSchedule,
         DateTime firstScheduledAt,
@@ -36,6 +61,26 @@ public static class BatchSchedulePlanner
             .ToList();
     }
 
+    public static IReadOnlyList<DateTime> BuildRecurringSchedule(
+        DateTime firstScheduledAt,
+        int sessionCount,
+        int intervalDays)
+    {
+        if (sessionCount is < 1 or > MaxTemplateSessionCount)
+        {
+            throw new ArgumentOutOfRangeException(nameof(sessionCount), sessionCount, "Session count must be between 1 and 52.");
+        }
+
+        if (intervalDays is < 1 or > MaxTemplateIntervalDays)
+        {
+            throw new ArgumentOutOfRangeException(nameof(intervalDays), intervalDays, "Interval must be between 1 and 365 days.");
+        }
+
+        return Enumerable.Range(0, sessionCount)
+            .Select(index => firstScheduledAt.AddDays(intervalDays * index))
+            .ToList();
+    }
+
     public static DateTime ShiftForClone(DateTime scheduledAt, BatchCloneInterval interval) =>
         interval switch
         {