v0.4.0: live feed updates (auto-insert new posts)

New artifacts appear at the top of the feed without a manual refresh.

- GET /feed/updates (session-authed): new artifacts since a timestamp marker,
  owner-scoped and filter-aware (source/label/kind/q), returned as row HTML.
- Feed JS polls every 5s, paused while the tab is hidden (immediate poll on
  refocus). Prepends new rows into today's group without touching the selected
  row or the preview iframe — what you're reading stays put and selected.
- Fresh rows show "now", aging into their real time after ~3 min; brief
  highlight on arrival.
- Extracted _artifact_rows.html partial, shared by the day-group render and the
  updates endpoint, so live rows match exactly.
- Polling chosen over SSE/WebSockets: zero infra on sync-WSGI + SQLite (see
  planning/next_steps for the SSE upgrade path). 142 tests pass; feed page +
  endpoint verified.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

artifacts/urls.py

@@ -6,6 +6,7 @@ from . import views
 urlpatterns = [
     path("", views.feed, name="feed"),
     path("feed/older", views.feed_older, name="feed-older"),
+    path("feed/updates", views.feed_updates, name="feed-updates"),
     path("a/<str:artifact_id>/read", views.mark_artifact_read, name="artifact-mark-read"),
     path("a/<str:artifact_id>/keep", views.toggle_artifact_keep, name="artifact-toggle-keep"),
     path("a/<str:artifact_id>/share", views.toggle_artifact_share, name="artifact-toggle-share"),