feat(webhook): add workflow_step webhook events for step-level CI notifications

Adds HookEventWorkflowStep event type that fires on every step state
transition (queued -> in_progress -> completed). Follows the same
pattern as the existing workflow_job events.

- New WorkflowStepPayload struct with run/job/step context
- WorkflowStepStatusUpdate notifier interface + dispatch
- Step state change detection in UpdateTask runner endpoint
- Fix: register workflow_step in updateHookEvents API mapping
- Full test coverage mirroring workflow_job tests

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

services/webhook/slack_test.go

@@ -155,6 +155,15 @@ func TestSlackPayload(t *testing.T) {
 
 		assert.Equal(t, "[<http://localhost:3000/test/repo|test/repo>] Release created: <http://localhost:3000/test/repo/releases/tag/v1.0|v1.0> by <https://try.gitea.io/user1|user1>", pl.Text)
 	})
+
+	t.Run("WorkflowStep", func(t *testing.T) {
+		p := workflowStepTestPayload()
+
+		pl, err := sc.WorkflowStep(p)
+		require.NoError(t, err)
+
+		assert.Equal(t, "Workflow Step completed: <http://localhost:3000/test/repo/actions/runs/42/jobs/1|build / Run tests:success> by <https://try.gitea.io/user1|user1>", pl.Text)
+	})
 }
 
 func TestSlackJSONPayload(t *testing.T) {