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

backend/avito_api/exceptions.py

@@ -0,0 +1,19 @@
+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