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>

static/agentfeed.css

@@ -1045,6 +1045,16 @@ kbd {
   color: var(--muted);
 }
 
+/* Newly live-inserted feed rows briefly highlight, then settle. */
+@keyframes agentfeed-row-arrive {
+  from { background: var(--surface-alt); }
+  to { background: transparent; }
+}
+
+.artifact-row.just-arrived {
+  animation: agentfeed-row-arrive 1.5s ease-out;
+}
+
 .api-key-create {
   align-items: end;
   display: flex;