fix: add validation for state_descr in sensor readings

Added validation to ensure state_descr:
- Is not empty when present (min: 1 character)
- Has reasonable length limit (max: 255 characters)
- Prevents storing invalid/malformed state descriptions

This prevents invalid state sensor readings from being stored.

Fixes Medium Bug #16 from reliability audit.

lib/towerops/snmp/sensor_reading.ex

@@ -41,6 +41,12 @@ defmodule Towerops.Snmp.SensorReading do
     |> cast(attrs, [:sensor_id, :value, :status, :state_descr, :checked_at])
     |> validate_required([:sensor_id, :status, :checked_at])
     |> validate_inclusion(:status, ["ok", "warning", "critical", "error"])
+    |> validate_state_descr()
     |> foreign_key_constraint(:sensor_id)
   end
+
+  # Validate state_descr when present - ensure it's not empty and has reasonable length
+  defp validate_state_descr(changeset) do
+    validate_length(changeset, :state_descr, min: 1, max: 255)
+  end
 end