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.

templates/partials/devices_table.html

@@ -85,6 +85,10 @@
                         <span class="px-2 py-1 rounded-full bg-purple-100 dark:bg-purple-900/30 text-purple-800 dark:text-purple-300 text-xs font-medium">
                             Modem
                         </span>
+                    {% elif unit.device_type == 'slm' %}
+                        <span class="px-2 py-1 rounded-full bg-orange-100 dark:bg-orange-900/30 text-orange-800 dark:text-orange-300 text-xs font-medium">
+                            SLM
+                        </span>
                     {% else %}
                         <span class="px-2 py-1 rounded-full bg-blue-100 dark:bg-blue-900/30 text-blue-800 dark:text-blue-300 text-xs font-medium">
                             Seismograph