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/admins.py

@@ -0,0 +1,9 @@
+from aiogram.types import InlineKeyboardButton, InlineKeyboardMarkup
+from aiogram.utils.keyboard import InlineKeyboardBuilder
+
+promotion_confirmation: InlineKeyboardMarkup = InlineKeyboardBuilder(
+    [
+        [InlineKeyboardButton(text="✅", callback_data="approve")],
+        [InlineKeyboardButton(text="❌", callback_data="decline")],
+    ]
+).as_markup()