feat: add StripeClient.update_subscription_price

lib/towerops/billing/stripe_client.ex

@@ -152,6 +152,25 @@ defmodule Towerops.Billing.StripeClient do
     post("/v1/prices", params)
   end
 
+  @doc """
+  Update subscription to use new price.
+
+  Fetches subscription to get item ID, then updates with proration_behavior: "none"
+  so price change takes effect at next billing cycle.
+  """
+  def update_subscription_price(subscription_id, new_price_id) do
+    with {:ok, sub} <- get_subscription(subscription_id) do
+      item_id = sub |> get_in(["items", "data"]) |> List.first() |> Map.get("id")
+
+      params = %{
+        items: [%{id: item_id, price: new_price_id}],
+        proration_behavior: "none"
+      }
+
+      post("/v1/subscriptions/#{subscription_id}", params)
+    end
+  end
+
   # HTTP Request Helpers
 
   defp get(path) do

test/towerops/billing/stripe_client_test.exs

@@ -360,4 +360,59 @@ defmodule Towerops.Billing.StripeClientTest do
       assert {:error, {:bad_request, "Invalid amount"}} = StripeClient.create_price("invalid")
     end
   end
+
+  describe "update_subscription_price/2" do
+    test "updates subscription to new price" do
+      # First stub: get subscription
+      Req.Test.stub(StripeClient, fn conn ->
+        if conn.method == "GET" and String.contains?(conn.request_path, "/v1/subscriptions/sub_test") do
+          conn
+          |> Plug.Conn.put_resp_content_type("application/json")
+          |> Plug.Conn.send_resp(
+            200,
+            Jason.encode!(%{
+              "id" => "sub_test",
+              "items" => %{
+                "data" => [%{"id" => "si_test123", "price" => %{"id" => "price_old"}}]
+              }
+            })
+          )
+        else
+          # Second stub: update subscription
+          {:ok, body, conn} = Plug.Conn.read_body(conn)
+          params = URI.decode_query(body)
+
+          assert params["items[0][id]"] == "si_test123"
+          assert params["items[0][price]"] == "price_new123"
+          assert params["proration_behavior"] == "none"
+
+          conn
+          |> Plug.Conn.put_resp_content_type("application/json")
+          |> Plug.Conn.send_resp(
+            200,
+            Jason.encode!(%{
+              "id" => "sub_test",
+              "items" => %{"data" => [%{"price" => %{"id" => "price_new123"}}]}
+            })
+          )
+        end
+      end)
+
+      assert {:ok, %{"id" => "sub_test"}} =
+               StripeClient.update_subscription_price("sub_test", "price_new123")
+    end
+
+    test "returns error when subscription not found" do
+      Req.Test.stub(StripeClient, fn conn ->
+        conn
+        |> Plug.Conn.put_resp_content_type("application/json")
+        |> Plug.Conn.send_resp(
+          404,
+          Jason.encode!(%{"error" => %{"message" => "No such subscription"}})
+        )
+      end)
+
+      assert {:error, _} = StripeClient.update_subscription_price("sub_invalid", "price_new")
+    end
+  end
 end