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`.

api/routes/pally.py

@@ -0,0 +1,78 @@
+import hashlib
+import logging
+import math
+
+from aiogram import Bot
+from fastapi import Depends, Form, HTTPException
+from fastapi.routing import APIRouter
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from api.core.deps import get_bot, get_db, get_deps
+from api.core.notifications import successful_payment_notification
+from config import settings
+from misc.pally import BillStatus
+from schemas.di import APIDependenciesDTO
+
+router = APIRouter(prefix="/checkout/pally")
+
+logger = logging.getLogger(__name__)
+
+
+@router.post("/callback")
+async def pally_callback(
+    id: str = Form(...),
+    bill_id: str = Form(...),
+    status: str = Form(...),
+    req_amount: str = Form(...),
+    currency: str = Form(...),
+    order_id: str | None = Form(None),
+    signature: str = Form(...),
+    session: AsyncSession = Depends(get_db),
+    bot: Bot = Depends(get_bot),
+    deps: APIDependenciesDTO = Depends(get_deps),
+):
+    oid = order_id or ""
+    raw_string = f"{id}{status}{req_amount}{currency}{oid}{settings.pally_token}"
+    md5_hash = hashlib.md5(raw_string.encode("utf-8")).hexdigest()
+    expected_signature = hashlib.sha1(md5_hash.encode("utf-8")).hexdigest()
+
+    if signature != expected_signature:
+        logger.critical("Invalid signature for bill %s", bill_id)
+        raise HTTPException(403, detail="Invalid signature.")
+
+    if status != BillStatus.SUCCESS or not req_amount.isdigit():
+        logger.info("Bill %s skipped: status=%s, req_amount=%s", bill_id, status, req_amount)
+        return "OK"
+
+    logger.info("Received successfully paid bill %s", bill_id)
+
+    if not oid.isdigit():
+        logger.critical("Invalid order_id format for bill %s", bill_id)
+        return "OK"
+
+    bill = await deps.bills_repository.get_bill_by_id(session, int(oid))
+
+    if not bill:
+        logger.critical("Bill %s is not found in db", bill_id)
+        return "OK"
+
+    try:
+        amount = int(req_amount)
+
+        await deps.user_repository.increase_user_balance(session, bill.creator_id, amount=amount)
+        await successful_payment_notification(bot, bill.creator_id, amount)
+
+        referal = bill.user.referal_id
+        if referal is not None:
+            referal_amount = math.floor(amount * (settings.referal_bonus / 100))
+            await deps.user_repository.increase_user_balance(session, referal, referal_amount)
+            await successful_payment_notification(bot, referal, referal_amount)
+
+    except Exception:
+        logger.exception(
+            "Exception caught while processing balances for bill %s (Creator: %s)",
+            bill_id,
+            bill.creator_id,
+        )
+
+    return "OK"