feat: standardize device type for Sound Level Meters (SLM)

- Updated all instances of device_type from "sound_level_meter" to "slm" across the codebase.
- Enhanced documentation to reflect the new device type standardization.
- Added migration script to convert legacy device types in the database.
- Updated relevant API endpoints, models, and frontend templates to use the new device type.
- Ensured backward compatibility by deprecating the old device type without data loss.

backend/routers/scheduler.py

@@ -131,7 +131,7 @@ async def create_scheduled_action(
         raise HTTPException(status_code=404, detail="Location not found")
 
     # Determine device type from location
-    device_type = "sound_level_meter" if location.location_type == "sound" else "seismograph"
+    device_type = "slm" if location.location_type == "sound" else "seismograph"
 
     # Get unit_id (optional - can be determined from assignment at execution time)
     unit_id = form_data.get("unit_id")
@@ -188,7 +188,7 @@ async def schedule_recording_session(
     if not location:
         raise HTTPException(status_code=404, detail="Location not found")
 
-    device_type = "sound_level_meter" if location.location_type == "sound" else "seismograph"
+    device_type = "slm" if location.location_type == "sound" else "seismograph"
     unit_id = form_data.get("unit_id")
 
     start_time = datetime.fromisoformat(form_data.get("start_time"))