feat: enhance roster unit management with bidirectional pairing sync

fix: scheduler one off

backend/services/slmm_client.py

@@ -112,6 +112,69 @@ class SLMMClient:
             error_msg = str(e) if str(e) else type(e).__name__
             raise SLMMClientError(f"Unexpected error: {error_msg}")
 
+    async def _download_request(
+        self,
+        endpoint: str,
+        data: Dict[str, Any],
+        unit_id: str,
+    ) -> Dict[str, Any]:
+        """
+        Make a download request to SLMM that returns binary file content (not JSON).
+
+        Saves the file locally and returns metadata about the download.
+        """
+        url = f"{self.api_base}{endpoint}"
+
+        try:
+            async with httpx.AsyncClient(timeout=httpx.Timeout(300.0)) as client:
+                response = await client.post(url, json=data)
+                response.raise_for_status()
+
+                # Determine filename from Content-Disposition header or generate one
+                content_disp = response.headers.get("content-disposition", "")
+                filename = None
+                if "filename=" in content_disp:
+                    filename = content_disp.split("filename=")[-1].strip('" ')
+
+                if not filename:
+                    remote_path = data.get("remote_path", "download")
+                    base = os.path.basename(remote_path.rstrip("/"))
+                    filename = f"{base}.zip" if not base.endswith(".zip") else base
+
+                # Save to local downloads directory
+                download_dir = os.path.join("data", "downloads", unit_id)
+                os.makedirs(download_dir, exist_ok=True)
+                local_path = os.path.join(download_dir, filename)
+
+                with open(local_path, "wb") as f:
+                    f.write(response.content)
+
+                return {
+                    "success": True,
+                    "local_path": local_path,
+                    "filename": filename,
+                    "size_bytes": len(response.content),
+                }
+
+        except httpx.ConnectError as e:
+            raise SLMMConnectionError(
+                f"Cannot connect to SLMM backend at {self.base_url}. "
+                f"Is SLMM running? Error: {str(e)}"
+            )
+        except httpx.HTTPStatusError as e:
+            error_detail = "Unknown error"
+            try:
+                error_data = e.response.json()
+                error_detail = error_data.get("detail", str(error_data))
+            except Exception:
+                error_detail = e.response.text or str(e)
+            raise SLMMDeviceError(f"SLMM download failed: {error_detail}")
+        except (SLMMConnectionError, SLMMDeviceError):
+            raise
+        except Exception as e:
+            error_msg = str(e) if str(e) else type(e).__name__
+            raise SLMMClientError(f"Download error: {error_msg}")
+
     # ========================================================================
     # Unit Management
     # ========================================================================
@@ -537,10 +600,13 @@ class SLMMClient:
             remote_path: Path on device to download (e.g., "/NL43_DATA/measurement.wav")
 
         Returns:
-            Binary file content (as response)
+            Dict with local_path, filename, size_bytes
         """
-        data = {"remote_path": remote_path}
-        return await self._request("POST", f"/{unit_id}/ftp/download", data=data)
+        return await self._download_request(
+            f"/{unit_id}/ftp/download",
+            {"remote_path": remote_path},
+            unit_id,
+        )
 
     async def download_folder(
         self,
@@ -557,10 +623,13 @@ class SLMMClient:
             remote_path: Folder path on device to download (e.g., "/NL43_DATA/Auto_0000")
 
         Returns:
-            Dict with local_path, folder_name, file_count, zip_size_bytes
+            Dict with local_path, folder_name, size_bytes
         """
-        data = {"remote_path": remote_path}
-        return await self._request("POST", f"/{unit_id}/ftp/download-folder", data=data)
+        return await self._download_request(
+            f"/{unit_id}/ftp/download-folder",
+            {"remote_path": remote_path},
+            unit_id,
+        )
 
     async def download_current_measurement(
         self,