fix: 3 bugs — notification type, referral with channel sub, BOT_USERNAME

1. Admin notification showed "renewal" instead of "first purchase" for new
   users because has_had_paid_subscription was set before notification.
   All 21 call sites now pass explicit purchase_type.

2. Partner referral not counted when mandatory channel subscription enabled.
   required_sub_channel_check saved campaign_id but not referrer_id from
   campaign.partner_user_id. Also removed duplicate DB query.

3. BOT_USERNAME auto-detection moved before web server start to close
   race window on /cabinet/branding/telegram-widget endpoint.

main.py

@@ -295,6 +295,11 @@ async def main():
             bot, dp = await setup_bot()
             stage.log('Кеш и FSM подготовлены')
 
+        bot_user = await bot.get_me()
+        if bot_user.username and not settings.BOT_USERNAME:
+            settings.BOT_USERNAME = bot_user.username
+            logger.info('BOT_USERNAME auto-detected', bot_username=bot_user.username)
+
         monitoring_service.bot = bot
         maintenance_service.set_bot(bot)
         broadcast_service.set_bot(bot)
@@ -516,7 +521,6 @@ async def main():
             success_message='Токен внешней админки готов',
         ) as stage:
             try:
-                bot_user = await bot.get_me()
                 token = await ensure_external_admin_token(
                     bot_user.username,
                     bot_user.id,