Add JSON response format to webhooks (#2671)

* Add JSON response format to webhooks

Support JSON responses for all webhook endpoints: index, show, create,
update, destroy, and activation. Adds jbuilder views, updates controllers
to use respond_to, and documents the API in docs/API.md.

* Address PR review feedback

- Use distinct board ID in API docs webhook example
- Use render "webhooks/show" in activations controller for consistency

* Address PR review feedback from flavorjones

- Include webhook.board in cache key to avoid stale board data
- Use _url instead of _path for location header consistency

* Fix webhook create test to match Location header format

Use board_webhook_url instead of board_webhook_path since the controller
returns a full URL in the Location header.

* Fix board ID in Location header example in API docs

Use distinct board ID in the Location URL to match the JSON body example.

test/controllers/webhooks/activations_controller_test.rb

@@ -32,4 +32,36 @@ class Webhooks::ActivationsControllerTest < ActionDispatch::IntegrationTest
     post board_webhook_activation_path(webhook.board, webhook)
     assert_response :forbidden
   end
+
+  test "create as JSON" do
+    webhook = webhooks(:inactive)
+
+    assert_not webhook.active?
+
+    assert_changes -> { webhook.reload.active? }, from: false, to: true do
+      post board_webhook_activation_path(webhook.board, webhook), as: :json
+    end
+
+    assert_response :created
+    assert_equal webhook.id, @response.parsed_body["id"]
+    assert_equal true, @response.parsed_body["active"]
+  end
+
+  test "cannot activate webhook on board without access as JSON" do
+    logout_and_sign_in_as :jason
+    webhook = webhooks(:inactive)
+
+    post board_webhook_activation_path(webhook.board, webhook), as: :json
+
+    assert_response :not_found
+  end
+
+  test "non-admin cannot activate webhook as JSON" do
+    logout_and_sign_in_as :jz
+    webhook = webhooks(:active)
+
+    post board_webhook_activation_path(webhook.board, webhook), as: :json
+
+    assert_response :forbidden
+  end
 end