fix: normalize MAC addresses with leading zeros for subscriber matching

When SNMP returns MAC addresses as strings (e.g., 'A:B:C:D:E:F'), they were
being converted to lowercase without padding hex digits with leading zeros.
This caused subscriber matching to fail because the normalize_mac function
expects properly padded MACs (e.g., '0a:0b:0c:0d:0e:0f').

Now properly normalizes string MACs by:
- Removing all separators
- Ensuring 12 hex characters
- Reformatting with colon separators and proper padding

This fixes subscriber matching for ePMP and some Ubiquiti devices that
return string-formatted MACs from SNMP.

lib/towerops/snmp/wireless_client_discovery.ex

@@ -253,9 +253,21 @@ defmodule Towerops.Snmp.WirelessClientDiscovery do
 
   defp format_mac(value) when is_binary(value) do
     if String.contains?(value, ":") or String.contains?(value, "-") do
+      # String MAC with separators - normalize to ensure proper padding
       value
       |> String.downcase()
-      |> String.replace("-", ":")
+      |> String.replace(~r/[^0-9a-f]/, "")
+      |> case do
+        <<a::binary-2, b::binary-2, c::binary-2, d::binary-2, e::binary-2, f::binary-2>> ->
+          "#{a}:#{b}:#{c}:#{d}:#{e}:#{f}"
+
+        # Not 12 hex chars - try to parse as byte string
+        _ ->
+          value
+          |> :binary.bin_to_list()
+          |> Enum.map_join(":", &String.pad_leading(Integer.to_string(&1, 16), 2, "0"))
+          |> String.downcase()
+      end
     else
       value
       |> :binary.bin_to_list()