fix: add missing sensor units to existing sensors

Sets sensor_unit for all sensors that are missing it, fixing sensors
created before the discovery code was updated to set units properly.

Updates:
- Temperature sensors: set to '°C'
- Voltage sensors: set to 'V'
- Current sensors: set to 'mA'
- Power sensors: set to 'W'
- dBm sensors: set to 'dBm'

This ensures the device detail page displays units correctly for all
sensor types (e.g., "27 °C" instead of "27" for temperature).

The discovery code already sets these units for new sensors, so this
migration only affects existing data.

Related to:
- lib/towerops/snmp/profiles/vendors/routeros.ex:690 (°C)
- lib/towerops/snmp/profiles/vendors/routeros.ex:709 (V)
- lib/towerops/snmp/profiles/base.ex:1473-1480 (sensor_type_to_unit)

🤖 Generated with Claude Code

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

priv/repo/migrations/20260125183652_add_missing_sensor_units.exs

@@ -0,0 +1,62 @@
+defmodule Towerops.Repo.Migrations.AddMissingSensorUnits do
+  use Ecto.Migration
+
+  def up do
+    # Set sensor_unit for all sensors that are missing it
+    # This fixes sensors created before sensor_unit was properly set in discovery code
+
+    # Temperature sensors
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_unit = '°C'
+    WHERE sensor_type = 'temperature'
+      AND (sensor_unit IS NULL OR sensor_unit = '')
+    """
+
+    # Voltage sensors (only those without unit set)
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_unit = 'V'
+    WHERE sensor_type = 'voltage'
+      AND (sensor_unit IS NULL OR sensor_unit = '')
+    """
+
+    # Current sensors
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_unit = 'mA'
+    WHERE sensor_type = 'current'
+      AND (sensor_unit IS NULL OR sensor_unit = '')
+    """
+
+    # Power sensors
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_unit = 'W'
+    WHERE sensor_type = 'power'
+      AND (sensor_unit IS NULL OR sensor_unit = '')
+    """
+
+    # dBm sensors (optical power)
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_unit = 'dBm'
+    WHERE sensor_type = 'dbm'
+      AND (sensor_unit IS NULL OR sensor_unit = '')
+    """
+
+    # State sensors (leave empty, they don't have units)
+    # Fan speed sensors should already have RPM
+    # Frequency, rate, etc. should already have their units
+  end
+
+  def down do
+    # Revert by setting units back to empty string
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_unit = ''
+    WHERE sensor_type IN ('temperature', 'voltage', 'current', 'power', 'dbm')
+      AND sensor_unit IN ('°C', 'V', 'mA', 'W', 'dBm')
+    """
+  end
+end