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.

README.md

@@ -308,7 +308,7 @@ print(response.json())
 |-------|------|-------------|
 | id | string | Unit identifier (primary key) |
 | unit_type | string | Hardware model name (default: `series3`) |
-| device_type | string | `seismograph` or `modem` discriminator |
+| device_type | string | Device type: `"seismograph"`, `"modem"`, or `"slm"` (sound level meter) |
 | deployed | boolean | Whether the unit is in the field |
 | retired | boolean | Removes the unit from deployments but preserves history |
 | note | string | Notes about the unit |
@@ -334,6 +334,39 @@ print(response.json())
 | phone_number | string | Cellular number for the modem |
 | hardware_model | string | Modem hardware reference |
 
+**Sound Level Meter (SLM) fields**
+
+| Field | Type | Description |
+|-------|------|-------------|
+| slm_host | string | Direct IP address for SLM (if not using modem) |
+| slm_tcp_port | integer | TCP control port (default: 2255) |
+| slm_ftp_port | integer | FTP file transfer port (default: 21) |
+| slm_model | string | Device model (NL-43, NL-53) |
+| slm_serial_number | string | Manufacturer serial number |
+| slm_frequency_weighting | string | Frequency weighting setting (A, C, Z) |
+| slm_time_weighting | string | Time weighting setting (F=Fast, S=Slow) |
+| slm_measurement_range | string | Measurement range setting |
+| slm_last_check | datetime | Last status check timestamp |
+| deployed_with_modem_id | string | Modem pairing (shared with seismographs) |
+
+### Device Type Schema
+
+Terra-View supports three device types with the following standardized `device_type` values:
+
+- **`"seismograph"`** (default) - Seismic monitoring devices (Series 3, Series 4, Micromate)
+  - Uses: calibration dates, modem pairing
+  - Examples: BE1234, UM12345 (Series 3/4 units)
+
+- **`"modem"`** - Field modems and network equipment
+  - Uses: IP address, phone number, hardware model
+  - Examples: MDM001, MODEM-2025-01
+
+- **`"slm"`** - Sound level meters (Rion NL-43/NL-53)
+  - Uses: TCP/FTP configuration, measurement settings, modem pairing
+  - Examples: SLM-43-01, NL43-001
+
+**Important**: All `device_type` values must be lowercase. The legacy value `"sound_level_meter"` has been deprecated in favor of the shorter `"slm"`. Run `backend/migrate_standardize_device_types.py` to update existing databases.
+
 ### Emitter Table (Device Check-ins)
 
 | Field | Type | Description |