fix: correct Mikrotik voltage sensor divisor from 1 to 10

Mikrotik/RouterOS voltage sensors were showing incorrect values (469.0
instead of 46.9) because existing sensors in the database have
sensor_divisor=1 instead of the correct value of 10.

The RouterOS profile code was fixed in a previous commit to set
divisor=10 for new sensors, but existing sensors need to be updated.

Migration changes:
- Updates sensor_divisor from 1 to 10 for Mikrotik voltage sensors
- Identifies Mikrotik sensors by OID prefix: 1.3.6.1.4.1.14988.1.1.3.100.1.3
- Divides all historical sensor_reading values by 10 to correct the data
- Both up and down migrations included for full reversibility

After this migration:
- Voltage will show as 46.9V instead of 469.0V
- Historical graphs will show correct voltage values

priv/repo/migrations/20260125180847_fix_mikrotik_voltage_sensor_divisor.exs

@@ -0,0 +1,64 @@
+defmodule Towerops.Repo.Migrations.FixMikrotikVoltageSensorDivisor do
+  use Ecto.Migration
+
+  def up do
+    # Fix voltage sensors for RouterOS/Mikrotik devices
+    # These should have sensor_divisor=10 (deci-volts) but were incorrectly set to 1
+
+    # Update voltage sensors where:
+    # 1. sensor_type = 'voltage'
+    # 2. sensor_unit = 'V'
+    # 3. sensor_divisor = 1 (incorrect)
+    # 4. OID starts with Mikrotik gauge table: 1.3.6.1.4.1.14988.1.1.3.100.1
+
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_divisor = 10
+    WHERE sensor_type = 'voltage'
+      AND sensor_unit = 'V'
+      AND sensor_divisor = 1
+      AND sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+    """
+
+    # Also divide existing sensor_reading values by 10 for these sensors
+    # This fixes historical data so graphs show correct values
+    execute """
+    UPDATE snmp_sensor_readings
+    SET value = value / 10
+    WHERE sensor_id IN (
+      SELECT id
+      FROM snmp_sensors
+      WHERE sensor_type = 'voltage'
+        AND sensor_unit = 'V'
+        AND sensor_divisor = 10
+        AND sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+    )
+    """
+  end
+
+  def down do
+    # Revert voltage sensors back to divisor=1
+    execute """
+    UPDATE snmp_sensors
+    SET sensor_divisor = 1
+    WHERE sensor_type = 'voltage'
+      AND sensor_unit = 'V'
+      AND sensor_divisor = 10
+      AND sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+    """
+
+    # 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_type = 'voltage'
+        AND sensor_unit = 'V'
+        AND sensor_divisor = 1
+        AND sensor_oid LIKE '1.3.6.1.4.1.14988.1.1.3.100.1.3.%'
+    )
+    """
+  end
+end