Replaced hard-coded 'processing' with 'gain_directory_name' value from MachineConfig in the demo API

Author: tieneupin

src/murfey/server/demo_api.py

@@ -1576,9 +1576,17 @@ async def process_gain(
     else:
         return {"gain_ref": None}
     gain_ref_out = (
-        (filepath / "processing" / f"gain_{gain_reference_params.tag}.mrc")
+        (
+            filepath
+            / machine_config.get("gain_directory_name", "processing")
+            / f"gain_{gain_reference_params.tag}.mrc"
+        )
         if gain_reference_params.tag
-        else (filepath / "processing" / "gain.mrc")
+        else (
+            filepath
+            / machine_config.get("gain_directory_name", "processing")
+            / "gain.mrc"
+        )
     )
     return {
         "gain_ref": gain_ref_out.relative_to(Path(machine_config["rsync_basepath"]))