feat: Implement payment notification system and API structure

- Added notifications for successful payments in `api/core/notifications.py`.
- Created main API entry point in `api/main.py` with FastAPI integration.
- Established routing structure in `api/routes/__init__.py` and `api/routes/pally.py` for handling payment callbacks.
- Developed billing handlers in `bot/handlers/billing.py` and `bot/handlers/buy.py` for subscription management.
- Introduced state management for user interactions in `bot/states/`.
- Created user interface elements in `bot/keyboards/` for navigation and payment processing.
- Set up middleware for dependency injection in `bot/middlewares/di.py`.
- Added support for user commands and interactions in `bot/handlers/common.py` and `bot/handlers/support.py`.
- Implemented logging and error handling throughout the bot's functionality.
- Created entry point script for API server in `entrypoints/api.sh`.

bot/handlers/billing.py

@@ -0,0 +1,49 @@
+import logging
+
+from aiogram import F, Router
+from aiogram.fsm.context import FSMContext
+from aiogram.types import CallbackQuery
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from bot.keyboards.client import pay_url, return_to_menu
+from bot.states.buy import BillingStorage
+from bot.texts import BILL_CREATED, BILL_CREATION, CALLBACK_FALLBACK
+from schemas.billing import BillingContext, BillingProviders
+from schemas.di import DependenciesDTO
+
+router = Router()
+logger = logging.getLogger(__name__)
+
+
+@router.callback_query(BillingStorage.pending, F.data == "billing:pally")
+async def pally_init(
+    cb: CallbackQuery, state: FSMContext, session: AsyncSession, deps: DependenciesDTO
+):
+    data = await state.get_data()
+    ctx: BillingContext | None = data.get("ctx")
+    if not ctx:
+        logger.warning("ctx not found on billing choice")
+        await cb.message.edit_text(CALLBACK_FALLBACK, reply_markup=return_to_menu)
+        return
+
+    await state.clear()
+
+    ctx.provider = BillingProviders.PALLY
+
+    await cb.message.edit_text(
+        BILL_CREATION.format(amount=ctx.amount, provider=ctx.provider.value.title())
+    )
+
+    bill = await deps.billing_service.pally_initiate_payment(
+        session, deps.pally_client, user_id=cb.from_user.id, amount=ctx.amount
+    )
+
+    if not bill:
+        await cb.message.edit_text(CALLBACK_FALLBACK, reply_markup=return_to_menu)
+        logger.critical("bill creation prompted, but bill is None")
+        return
+
+    await cb.message.edit_text(
+        BILL_CREATED.format(amount=ctx.amount, provider=ctx.provider.value.title()),
+        reply_markup=pay_url(bill.link_page_url),
+    )