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/discord_test.go

@@ -280,6 +280,21 @@ func TestDiscordPayload(t *testing.T) {
 		assert.Equal(t, setting.AppURL+p.Sender.UserName, pl.Embeds[0].Author.URL)
 		assert.Equal(t, p.Sender.AvatarURL, pl.Embeds[0].Author.IconURL)
 	})
+
+	t.Run("WorkflowStep", func(t *testing.T) {
+		p := workflowStepTestPayload()
+
+		pl, err := dc.WorkflowStep(p)
+		require.NoError(t, err)
+
+		assert.Len(t, pl.Embeds, 1)
+		assert.Equal(t, "Workflow Step completed: build / Run tests:success", pl.Embeds[0].Title)
+		assert.Empty(t, pl.Embeds[0].Description)
+		assert.Equal(t, p.WorkflowJob.HTMLURL, pl.Embeds[0].URL)
+		assert.Equal(t, p.Sender.UserName, pl.Embeds[0].Author.Name)
+		assert.Equal(t, setting.AppURL+p.Sender.UserName, pl.Embeds[0].Author.URL)
+		assert.Equal(t, p.Sender.AvatarURL, pl.Embeds[0].Author.IconURL)
+	})
 }
 
 func TestDiscordJSONPayload(t *testing.T) {