fix(critical): ensure all money math uses Decimal, never floating point

CRITICAL FINANCIAL BUGS FIXED:

1. Stripe price creation was sending dollars instead of cents
   - create_price() now converts $2.00 -> 200 cents
   - Was sending "2.00" to unit_amount_decimal (0.02 dollars!)
   - Now correctly sends "200.00" (2 dollars in cents)

2. VISP sync was using floating point arithmetic for MRR
   - calculate_total_mrr() was using 0.0 accumulator and float math
   - Now uses Decimal.new("0") and Decimal.add() throughout
   - Prevents precision loss when summing subscriber MRR

All money calculations now use Decimal library to avoid floating
point precision errors. Amounts are only converted to cents (integers)
at the boundary when sending to Stripe API.

lib/towerops/billing/stripe_client.ex

@@ -130,13 +130,24 @@ defmodule Towerops.Billing.StripeClient do
   @doc """
   Create a new Price object for metered billing.
 
+  Accepts price in dollars (e.g. "2.00") and converts to cents for Stripe API.
+  Stripe's unit_amount_decimal expects cents, not dollars.
+
   Returns {:ok, price_object} or {:error, reason}.
   """
-  def create_price(unit_amount_decimal) when is_binary(unit_amount_decimal) do
+  def create_price(unit_amount_dollars) when is_binary(unit_amount_dollars) do
+    # Convert dollars to cents (Stripe expects cents)
+    # "2.00" -> Decimal.new("2.00") -> Decimal.mult(100) -> "200.00"
+    unit_amount_cents =
+      unit_amount_dollars
+      |> Decimal.new()
+      |> Decimal.mult(Decimal.new("100"))
+      |> Decimal.to_string()
+
     params = %{
       product: stripe_product_id(),
       currency: "usd",
-      unit_amount_decimal: unit_amount_decimal,
+      unit_amount_decimal: unit_amount_cents,
       recurring: %{
         interval: "month",
         usage_type: "metered",

lib/towerops/visp/sync.ex

@@ -31,10 +31,10 @@ defmodule Towerops.Visp.Sync do
 
       message =
         "Synced #{subscribers_count} subscribers, #{sites_count} sites, #{equipment_count} equipment" <>
-          if(total_mrr > 0, do: " (MRR: $#{:erlang.float_to_binary(total_mrr / 1, decimals: 2)})", else: "")
+          if(Decimal.compare(total_mrr, "0") == :gt, do: " (MRR: $#{Decimal.round(total_mrr, 2)})", else: "")
 
       Integrations.update_sync_status(integration, "success", message)
-      Integrations.update_billing_totals(integration, subscribers_count, Decimal.from_float(total_mrr))
+      Integrations.update_billing_totals(integration, subscribers_count, total_mrr)
 
       {:ok,
        %{
@@ -50,12 +50,22 @@ defmodule Towerops.Visp.Sync do
   end
 
   defp calculate_total_mrr(subscribers) do
-    Enum.reduce(subscribers, 0.0, fn sub, acc ->
-      mrr = sub["mrr"]
-      if is_number(mrr), do: acc + mrr, else: acc
+    Enum.reduce(subscribers, Decimal.new("0"), fn sub, acc ->
+      add_subscriber_mrr(sub["mrr"], acc)
     end)
   end
 
+  defp add_subscriber_mrr(mrr, acc) when is_float(mrr) do
+    # Convert float to Decimal to avoid floating point math
+    Decimal.add(acc, Decimal.from_float(mrr))
+  end
+
+  defp add_subscriber_mrr(mrr, acc) when is_integer(mrr) do
+    Decimal.add(acc, Decimal.new(mrr))
+  end
+
+  defp add_subscriber_mrr(_mrr, acc), do: acc
+
   defp humanize_sync_error(:unauthorized), do: "Authentication failed — check your API key"
   defp humanize_sync_error(:forbidden), do: "Access denied — your API key may not have sufficient permissions"
   defp humanize_sync_error({:rate_limited, retry_after}), do: "Rate limited by VISP — retry after #{retry_after}s"