Use dom-on for event handlers; add CI config and stepper Playwright test

- web/orchestration.sx, web/signals.sx: dom-listen → dom-on (trampoline
  wrapper that resolves TCO thunks from Lambda event handlers)
- .gitea/: CI workflow and Dockerfile for automated test runs
- tests/playwright/stepper.spec.js: stepper widget smoke test
- Remove stale artdag .pyc file

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

web/signals.sx

@@ -425,7 +425,7 @@
 
 (define on-event :effects [io]
   (fn (el (event-name :as string) (handler :as lambda))
-    (dom-listen el event-name handler)))
+    (dom-on el event-name handler)))
 
 ;; Convenience: create an effect that listens for a DOM event on an
 ;; element and writes the event detail (or a transformed value) into
@@ -436,7 +436,7 @@
 (define bridge-event :effects [mutation io]
   (fn (el (event-name :as string) (target-signal :as signal) transform-fn)
     (effect (fn ()
-      (let ((remove (dom-listen el event-name
+      (let ((remove (dom-on el event-name
               (fn (e)
                 (let ((detail (event-detail e))
                       (new-val (if transform-fn