fix(discord): resolve permission checking for /newsession command

- DiscordPermissionChecker: removed dead-code userRoles overload;
  now only uses resolvedPermissions bitflag (Administrator = 0x8).
- DiscordNewSessionCommand: computes resolved permissions from guild
  user roles via Context.Guild.Users[Id].RoleIds + guild.Roles.
- DiscordNewSessionHandler: updated signature to accept ulong
  resolvedPermissions instead of unused userRoles.
- Added ILogger to command for diagnostics on unexpected errors.
- Added test: regular user with ManageServer (but not Admin) is rejected.

Refs issue #28

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

src/GmRelay.DiscordBot/Features/Sessions/DiscordNewSessionHandler.cs

@@ -51,7 +51,7 @@ public sealed class DiscordNewSessionHandler(
         string channelId,
         ulong userId,
         string userDisplayName,
-        IEnumerable<ulong> userRoles,
+        ulong resolvedPermissions,
         ulong guildOwnerId,
         string title,
         DateTimeOffset scheduledAt,
@@ -69,7 +69,7 @@ public sealed class DiscordNewSessionHandler(
               WHERE g.platform = 'Discord' AND g.external_group_id = @GuildId",
             new { GuildId = guildId });
 
-        if (!permissionChecker.CanManageSchedule(guildOwnerId, userId, userRoles, dbManagerUserIds))
+        if (!permissionChecker.CanManageSchedule(guildOwnerId, userId, dbManagerUserIds, resolvedPermissions))
         {
             throw new UnauthorizedAccessException("⛔ Только owner, администратор или manager могут создавать сессии.");
         }