add Printful webhook endpoint with token verification

New POST /webhooks/printful route with VerifyPrintfulWebhook plug
(shared secret token via header or query param). Handles package_shipped,
order_failed, order_canceled, product_updated, product_synced, and
product_deleted events. Webhook registration via Printful v2 API with
token appended to URL. 19 new tests (819 total).

Also marks task #28 as done — Printful sync products already include
preview mockup images handled by the existing ImageDownloadWorker
pipeline.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

test/simpleshop_theme_web/controllers/webhook_controller_test.exs

@@ -98,4 +98,81 @@ defmodule SimpleshopThemeWeb.WebhookControllerTest do
     :crypto.mac(:hmac, :sha256, secret, body)
     |> Base.encode16(case: :lower)
   end
+
+  describe "POST /webhooks/printful" do
+    @printful_secret "printful_test_secret_456"
+
+    setup do
+      _conn =
+        provider_connection_fixture(%{
+          provider_type: "printful",
+          config: %{"store_id" => "99999", "webhook_secret" => @printful_secret}
+        })
+
+      :ok
+    end
+
+    test "returns 401 without token", %{conn: conn} do
+      conn =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> post(~p"/webhooks/printful", %{type: "product_updated"})
+
+      assert json_response(conn, 401)["error"] == "Invalid token"
+    end
+
+    test "returns 401 with wrong token", %{conn: conn} do
+      conn =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> put_req_header("x-pf-webhook-token", "wrong_token")
+        |> post(~p"/webhooks/printful", %{type: "product_updated"})
+
+      assert json_response(conn, 401)["error"] == "Invalid token"
+    end
+
+    test "accepts valid token via header", %{conn: conn} do
+      conn =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> put_req_header("x-pf-webhook-token", @printful_secret)
+        |> post(~p"/webhooks/printful", %{type: "product_updated", data: %{}})
+
+      assert json_response(conn, 200)["status"] == "ok"
+    end
+
+    test "accepts valid token via query param", %{conn: conn} do
+      conn =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> post(~p"/webhooks/printful?token=#{@printful_secret}", %{
+          type: "product_updated",
+          data: %{}
+        })
+
+      assert json_response(conn, 200)["status"] == "ok"
+    end
+
+    test "handles unknown event gracefully", %{conn: conn} do
+      conn =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> put_req_header("x-pf-webhook-token", @printful_secret)
+        |> post(~p"/webhooks/printful", %{type: "unknown_event", data: %{}})
+
+      assert json_response(conn, 200)["status"] == "ok"
+    end
+
+    test "returns 401 when no webhook secret configured", %{conn: conn} do
+      SimpleshopTheme.Repo.delete_all(SimpleshopTheme.Products.ProviderConnection)
+
+      conn =
+        conn
+        |> put_req_header("content-type", "application/json")
+        |> put_req_header("x-pf-webhook-token", @printful_secret)
+        |> post(~p"/webhooks/printful", %{type: "product_updated"})
+
+      assert json_response(conn, 401)["error"] == "Invalid token"
+    end
+  end
 end