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/roster_edit.py

@@ -223,7 +223,7 @@ async def add_roster_unit(
     db.commit()
 
     # If sound level meter, sync config to SLMM cache
-    if device_type == "sound_level_meter":
+    if device_type == "slm":
         logger.info(f"Syncing SLM {id} config to SLMM cache...")
         result = await sync_slm_to_slmm_cache(
             unit_id=id,