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

backend/ozon_api/exceptions.py

@@ -0,0 +1,15 @@
+class OzonAPIError(Exception):
+    """Base exception for all Ozon API errors."""
+    pass
+
+class OzonAuthError(OzonAPIError):
+    """Raised when authentication fails (e.g. invalid Client-Id or Api-Key)."""
+    pass
+
+class OzonRateLimitError(OzonAPIError):
+    """Raised when API rate limits are exceeded (HTTP 429)."""
+    pass
+
+class OzonBadRequestError(OzonAPIError):
+    """Raised when API returns HTTP 400 (Bad Request)."""
+    pass