fix: add detailed error logging for ARP upsert failures

Previously just logged a count without any details, making debugging
impossible. Now logs first 3 sample errors with IP, MAC, and validation
errors to help identify root cause.

Sample output:
  290 ARP upsert failures for device X
  sample_errors: [
    %{ip: nil, mac: "00:11:22:33:44:55", errors: %{ip_address: ["can't be blank"]}},
    %{ip: "10.0.0.1", mac: nil, errors: %{mac_address: ["can't be blank"]}},
    ...
  ]

lib/towerops/snmp.ex

@@ -1858,6 +1858,26 @@ defmodule Towerops.Snmp do
     end)
   end
 
+  # Helper to format changeset errors for logging
+  defp format_changeset_errors(changeset) do
+    Ecto.Changeset.traverse_errors(changeset, fn {msg, _opts} -> msg end)
+  end
+
+  # Helper to collect error samples (first N errors for debugging)
+  defp maybe_add_error_sample(error_count, samples, attrs, changeset, max_samples \\ 3) do
+    if error_count < max_samples do
+      error_details = %{
+        ip: Map.get(attrs, :ip_address),
+        mac: Map.get(attrs, :mac_address),
+        errors: format_changeset_errors(changeset)
+      }
+
+      [error_details | samples]
+    else
+      samples
+    end
+  end
+
   @doc """
   Bulk upserts ARP entries for a device.
   Returns `{success_count, error_count}` tuple.
@@ -1866,7 +1886,7 @@ defmodule Towerops.Snmp do
     interface_map = Map.new(interfaces, fn i -> {i.if_index, i.id} end)
 
     arp_entries
-    |> Enum.reduce({0, 0}, fn entry, {s, e} ->
+    |> Enum.reduce({0, 0, []}, fn entry, {s, e, error_samples} ->
       interface_id = Map.get(interface_map, entry.if_index)
 
       attrs =
@@ -1875,18 +1895,25 @@ defmodule Towerops.Snmp do
         |> Map.put(:interface_id, interface_id)
 
       case upsert_arp_entry(attrs) do
-        {:ok, _} -> {s + 1, e}
-        {:error, _} -> {s, e + 1}
+        {:ok, _} ->
+          {s + 1, e, error_samples}
+
+        {:error, changeset} ->
+          # Sample first 3 errors for debugging
+          sample = maybe_add_error_sample(e, error_samples, attrs, changeset)
+          {s, e + 1, sample}
       end
     end)
-    |> tap(fn {_s, errors} ->
+    |> tap(fn {_s, errors, error_samples} ->
       if errors > 0 do
         Logger.warning("#{errors} ARP upsert failures for device #{device_id}",
           device_id: device_id,
-          error_count: errors
+          error_count: errors,
+          sample_errors: Enum.reverse(error_samples)
         )
       end
     end)
+    |> then(fn {s, e, _samples} -> {s, e} end)
   end
 
   # MAC Address queries