[backend/masto-client] Fix datetime string handling

Iceshrimp.Backend/Controllers/Mastodon/Renderers/NoteRenderer.cs

@@ -74,8 +74,8 @@ public class NoteRenderer(
 			Renote         = renote, //TODO: check if it's a pure renote
 			Quote          = renote, //TODO: see above
 			ContentType    = "text/x.misskeymarkdown",
-			CreatedAt      = note.CreatedAt.ToString("O")[..^5],
-			EditedAt       = note.UpdatedAt?.ToString("O")[..^5],
+			CreatedAt      = note.CreatedAt.ToStringMastodon(),
+			EditedAt       = note.UpdatedAt?.ToStringMastodon(),
 			RepliesCount   = note.RepliesCount,
 			RenoteCount    = note.RenoteCount,
 			FavoriteCount  = 0,     //FIXME

Iceshrimp.Backend/Controllers/Mastodon/Renderers/NotificationRenderer.cs

@@ -26,7 +26,7 @@ public class NotificationRenderer(NoteRenderer noteRenderer, UserRenderer userRe
 			Type      = Notification.EncodeType(notification.Type),
 			Note      = note,
 			Notifier  = notifier,
-			CreatedAt = notification.CreatedAt.ToString("O")[..^5]
+			CreatedAt = notification.CreatedAt.ToStringMastodon()
 		};
 
 		return res;

Iceshrimp.Backend/Controllers/Mastodon/Renderers/UserRenderer.cs

@@ -23,7 +23,7 @@ public class UserRenderer(IOptions<Config.InstanceSection> config, MfmConverter
 			Acct = acct,
 			FullyQualifiedName = $"{user.Username}@{user.Host ?? config.Value.AccountDomain}",
 			IsLocked = user.IsLocked,
-			CreatedAt = user.CreatedAt.ToString("O")[..^5],
+			CreatedAt = user.CreatedAt.ToStringMastodon(),
 			FollowersCount = user.FollowersCount,
 			FollowingCount = user.FollowingCount,
 			StatusesCount = user.NotesCount,

Iceshrimp.Backend/Core/Extensions/DateTimeExtensions.cs

@@ -0,0 +1,7 @@
+namespace Iceshrimp.Backend.Core.Extensions;
+
+public static class DateTimeExtensions {
+	public static string ToStringMastodon(this DateTime dateTime) {
+		return dateTime.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fffK");
+	}
+}