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.

sync_slms_to_slmm.py

@@ -25,7 +25,7 @@ async def sync_all_slms():
     try:
         # Get all SLM devices from Terra-View (source of truth)
         slm_devices = db.query(RosterUnit).filter_by(
-            device_type="sound_level_meter"
+            device_type="slm"
         ).all()
 
         logger.info(f"Found {len(slm_devices)} SLM devices in Terra-View roster")