feat: implement backend API clients for multi-marketplace integration and frontend service layer

backend/avito_api/__init__.py

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+from .client import AvitoClient
+from .services.items import ItemsService
+from .services.messenger import MessengerService
+
+class AvitoAPI:
+    """
+    Main entry point for Avito SDK.
+    Instantiates all services with a shared client.
+    """
+    def __init__(self, client_id: str = None, client_secret: str = None):
+        self.client = AvitoClient(client_id, client_secret)
+        
+        # Initialize services
+        self.items = ItemsService(self.client)
+        self.messenger = MessengerService(self.client)

backend/avito_api/client.py

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+import os
+import requests
+import time
+from typing import Dict, Any, Optional
+from .exceptions import AvitoAPIError, AvitoAuthError, AvitoRateLimitError, AvitoBadRequestError, AvitoNotFoundError
+
+class AvitoClient:
+    """Base client for interacting with Avito API."""
+    
+    BASE_URL = "https://api.avito.ru"
+    
+    def __init__(self, client_id: Optional[str] = None, client_secret: Optional[str] = None):
+        # Fetch from env if not provided
+        self.client_id = client_id or os.getenv('AVITO_CLIENT_ID')
+        self.client_secret = client_secret or os.getenv('AVITO_CLIENT_SECRET')
+        
+        if not self.client_id or not self.client_secret:
+            raise ValueError("AVITO_CLIENT_ID and AVITO_CLIENT_SECRET must be provided")
+            
+        self.session = requests.Session()
+        self.access_token = None
+        
+        # Получаем токен сразу при инициализации
+        self._authenticate()
+
+    def _authenticate(self):
+        """Exchange Client ID and Secret for a temporary OAuth access token."""
+        url = f"{self.BASE_URL}/token/"
+        data = {
+            "client_id": self.client_id,
+            "client_secret": self.client_secret,
+            "grant_type": "client_credentials"
+        }
+        
+        response = requests.post(url, data=data)
+        if response.status_code != 200:
+            raise AvitoAuthError(f"Failed to get Avito token: {response.text}")
+            
+        token_data = response.json()
+        self.access_token = token_data.get("access_token")
+        
+        self.session.headers.update({
+            "Authorization": f"Bearer {self.access_token}",
+            "Content-Type": "application/json"
+        })
+
+    def request(self, method: str, path: str, data: Optional[Dict[str, Any]] = None, params: Optional[Dict[str, Any]] = None, retries: int = 3) -> Dict[str, Any]:
+        """Perform HTTP request to Avito API with automatic retries and token refresh."""
+        url = f"{self.BASE_URL}{path}"
+        
+        for attempt in range(retries):
+            try:
+                response = self.session.request(method, url, json=data, params=params)
+                
+                # Check for expired token (401) or Forbidden (403)
+                if response.status_code in (401, 403):
+                    if attempt < retries - 1:
+                        # Обновляем токен и пробуем снова
+                        self._authenticate()
+                        continue
+                    raise AvitoAuthError(f"Authentication failed after retries: {response.text}")
+                
+                # Check rate limiting (429)
+                if response.status_code == 429:
+                    if attempt < retries - 1:
+                        time.sleep(2 ** attempt)  # Exponential backoff
+                        continue
+                    raise AvitoRateLimitError("Rate limit exceeded")
+                    
+                if response.status_code == 400:
+                    raise AvitoBadRequestError(f"Bad Request: {response.text}")
+
+                if response.status_code == 404:
+                    raise AvitoNotFoundError(f"Not Found: {response.text}")
+                    
+                response.raise_for_status()
+                
+                if response.content:
+                    return response.json()
+                return {}
+
+            except requests.exceptions.RequestException as e:
+                if attempt < retries - 1:
+                    time.sleep(2 ** attempt)
+                    continue
+                raise AvitoAPIError(f"Request failed: {str(e)}")
+                
+        raise AvitoAPIError("Request failed after max retries")
+
+    def post(self, path: str, data: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
+        return self.request("POST", path, data=data)
+
+    def get(self, path: str, params: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
+        return self.request("GET", path, params=params)
+
+    def put(self, path: str, data: Optional[Dict[str, Any]] = None) -> Dict[str, Any]:
+        return self.request("PUT", path, data=data)

backend/avito_api/exceptions.py

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+class AvitoAPIError(Exception):
+    """Base exception for all Avito API errors."""
+    pass
+
+class AvitoAuthError(AvitoAPIError):
+    """Raised when authentication fails (HTTP 401 or 403)."""
+    pass
+
+class AvitoRateLimitError(AvitoAPIError):
+    """Raised when API rate limits are exceeded (HTTP 429)."""
+    pass
+
+class AvitoBadRequestError(AvitoAPIError):
+    """Raised when API returns HTTP 400."""
+    pass
+
+class AvitoNotFoundError(AvitoAPIError):
+    """Raised when API returns HTTP 404."""
+    pass

backend/avito_api/services/items.py

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+from typing import Dict, Any, Optional
+from ..client import AvitoClient
+
+class ItemsService:
+    """Service to handle Avito Items (Listings)."""
+    
+    def __init__(self, client: AvitoClient):
+        self.client = client
+
+    def get_info(self, item_id: str) -> Dict[str, Any]:
+        """
+        Get info about a specific item.
+        Endpoint: /core/v1/items/{item_id}
+        """
+        return self.client.get(f"/core/v1/items/{item_id}")

backend/avito_api/services/messenger.py

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+from typing import Dict, Any, Optional
+from ..client import AvitoClient
+
+class MessengerService:
+    """Service to handle Avito Messenger (Chats)."""
+    
+    def __init__(self, client: AvitoClient):
+        self.client = client
+
+    def get_chats(self, user_id: str, limit: int = 100, offset: int = 0) -> Dict[str, Any]:
+        """
+        Get list of active chats for a user.
+        Endpoint: /messenger/v2/accounts/{user_id}/chats
+        """
+        params = {"limit": limit, "offset": offset}
+        return self.client.get(f"/messenger/v2/accounts/{user_id}/chats", params=params)
+
+    def send_message(self, user_id: str, chat_id: str, text: str) -> Dict[str, Any]:
+        """
+        Send a text message to a specific chat.
+        Endpoint: /messenger/v1/accounts/{user_id}/chats/{chat_id}/messages
+        """
+        payload = {
+            "message": {
+                "text": text
+            },
+            "type": "text"
+        }
+        return self.client.post(f"/messenger/v1/accounts/{user_id}/chats/{chat_id}/messages", data=payload)