fix(imports): handle DateTime microseconds and integer string floats

Fixed two import-related errors:

1. DateTime microseconds error when updating API token last_used_at:
   - Error: :utc_datetime expects microseconds to be empty
   - Fix: Use DateTime.truncate(:second) to remove microseconds
   - Location: lib/towerops/api_tokens.ex:179

2. Float parsing error for integer strings like "0":
   - Error: :erlang.binary_to_float("0") - not a textual representation of float
   - Fix: Use Float.parse/1 instead of String.to_float/1
   - Float.parse/1 handles both "0" and "0.0" correctly
   - Returns {float, remainder} on success, :error on failure
   - Location: lib/towerops/device_profiles/importer.ex:380

Both errors were causing profile imports to fail with crashes.

lib/towerops/api_tokens.ex

@@ -176,7 +176,7 @@ defmodule Towerops.ApiTokens do
     # We don't care if this fails
     Task.start(fn ->
       token
-      |> Ecto.Changeset.change(last_used_at: DateTime.utc_now())
+      |> Ecto.Changeset.change(last_used_at: DateTime.truncate(DateTime.utc_now(), :second))
       |> Repo.update()
     end)
   end

lib/towerops/device_profiles/importer.ex

@@ -377,7 +377,14 @@ defmodule Towerops.DeviceProfiles.Importer do
   defp parse_float(nil), do: nil
   defp parse_float(value) when is_float(value), do: value
   defp parse_float(value) when is_integer(value), do: value * 1.0
-  defp parse_float(value) when is_binary(value), do: String.to_float(value)
+
+  defp parse_float(value) when is_binary(value) do
+    case Float.parse(value) do
+      {float, _remainder} -> float
+      :error -> nil
+    end
+  end
+
   defp parse_float(_), do: nil
 
   defp extract_sensor_options(sensor) do