fix: correct misclassified MikroTik voltage sensors

Fixes voltage sensors that were incorrectly classified as temperature
sensors due to MikroTik devices reporting wrong mtxrGaugeUnit values.

Problem:
- Sensors with names like "psu1-voltage" and "psu2-voltage" were
  classified as temperature sensors with divisor=1
- This caused voltage readings to display as 469V instead of 46.9V
- The RouterOS profile already has name-based override logic for new
  discoveries, but existing sensors needed fixing

Solution:
- Migration identifies sensors by:
  - OID pattern: 1.3.6.1.4.1.14988.1.1.3.100.1.3.%
  - Type mismatch: sensor_type = 'temperature' but name contains 'voltage'
  - Incorrect divisor: sensor_divisor = 1
- Updates sensor metadata: type, unit, divisor
- Corrects historical sensor_reading values by dividing by 10

Future discoveries are already protected by the name-based override
logic in lib/towerops/snmp/profiles/vendors/routeros.ex:620-630

🤖 Generated with Claude Code

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

priv/repo/migrations/20260125182836_fix_misclassified_mikrotik_voltage_sensors.exs

@@ -0,0 +1,63 @@
+defmodule Towerops.Repo.Migrations.FixMisclassifiedMikrotikVoltageSensors do
+  use Ecto.Migration
+
+  def up do
+    # Fix voltage sensors that were incorrectly classified as temperature
+    # These sensors have "voltage" in their name but were classified as temperature
+    # This happens when MikroTik devices report incorrect mtxrGaugeUnit values
+
+    # Update sensors: set correct type, unit, and divisor
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_type = 'voltage',
+        sensor_unit = 'V',
+        sensor_divisor = 10
+    WHERE sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+      AND sensor_type = 'temperature'
+      AND LOWER(sensor_descr) LIKE '%voltage%'
+      AND sensor_divisor = 1
+    """
+
+    # Divide existing sensor_reading values by 10 to correct historical data
+    execute """
+    UPDATE snmp_sensor_readings
+    SET value = value / 10
+    WHERE sensor_id IN (
+      SELECT id
+      FROM snmp_sensors
+      WHERE sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+        AND sensor_type = 'voltage'
+        AND LOWER(sensor_descr) LIKE '%voltage%'
+        AND sensor_divisor = 10
+    )
+    """
+  end
+
+  def down do
+    # Revert changes
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_type = 'temperature',
+        sensor_unit = '',
+        sensor_divisor = 1
+    WHERE sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+      AND sensor_type = 'voltage'
+      AND LOWER(sensor_descr) LIKE '%voltage%'
+      AND sensor_divisor = 10
+    """
+
+    # Multiply sensor readings back by 10
+    execute """
+    UPDATE snmp_sensor_readings
+    SET value = value * 10
+    WHERE sensor_id IN (
+      SELECT id
+      FROM snmp_sensors
+      WHERE sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+        AND sensor_type = 'temperature'
+        AND LOWER(sensor_descr) LIKE '%voltage%'
+        AND sensor_divisor = 1
+    )
+    """
+  end
+end