feat: add admin promotion broadcast system and psql health check

- Add `/push` command for admins to broadcast messages to all users
  with approve/decline confirmation flow (handlers/ads.py)
- Add common handler for undefined commands and delete callback
- Add `UserRepository.get_users()` to fetch all users
- Add `UserService.send_promotion_to_users()` with error logging
- Add promotion confirmation keyboard and FSM state
- Add PostgreSQL availability check in startup.sh before migrations
- Add new text constants for promotion flow

keyboards/client.py

@@ -38,6 +38,10 @@ def _return_btn(data: str = "menu:main", text: str = "⬅️ Назад") -> Inl
     return InlineKeyboardButton(text=text, callback_data=data)
 
 
+def build_return_menu(data: str = "menu:main", text: str = "⬅️ Назад") -> InlineKeyboardMarkup:
+    return InlineKeyboardBuilder([[_return_btn(data, text)]]).as_markup()
+
+
 def devices_selector(current: int = settings.min_devices, whitelists: bool = False):
     builder = InlineKeyboardBuilder()