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

backend/mm_api/exceptions.py

@@ -0,0 +1,19 @@
+class MMAPIError(Exception):
+    """Base exception for all Magnit Market API errors."""
+    pass
+
+class MMAuthError(MMAPIError):
+    """Raised when authentication fails (HTTP 401 or 403)."""
+    pass
+
+class MMRateLimitError(MMAPIError):
+    """Raised when API rate limits are exceeded (HTTP 429)."""
+    pass
+
+class MMBadRequestError(MMAPIError):
+    """Raised when API returns HTTP 400 (Bad Request)."""
+    pass
+
+class MMNotFoundError(MMAPIError):
+    """Raised when API returns HTTP 404 (Not Found)."""
+    pass