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/projects.py

@@ -549,7 +549,7 @@ async def get_ftp_browser(
         location = db.query(MonitoringLocation).filter_by(id=assignment.location_id).first()
 
         # Only include SLM units
-        if unit and unit.device_type == "sound_level_meter":
+        if unit and unit.device_type == "slm":
             units_data.append({
                 "assignment": assignment,
                 "unit": unit,