fix: brain polling — update individual items instead of full reload

Polls only pending/processing items by ID every 4s instead of
reloading the entire list every 3s. Prevents screen flashing.

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

frontend-v2/src/lib/pages/brain/AtelierBrainPage.svelte

@@ -217,25 +217,37 @@
   function startPolling() {
     stopPolling();
     pollTimer = setInterval(async () => {
-      const hasPending = items.some(i => i.processing_status === 'pending' || i.processing_status === 'processing');
-      if (hasPending) {
-        await loadItems();
-        // Update selected item if it was pending
-        if (selectedItem) {
-          const updated = items.find(i => i.id === selectedItem!.id);
-          if (updated) selectedItem = updated;
-        }
-      } else {
+      const pendingIds = items
+        .filter(i => i.processing_status === 'pending' || i.processing_status === 'processing')
+        .map(i => i.id);
+
+      if (pendingIds.length === 0) {
         stopPolling();
+        return;
       }
-    }, 3000);
+
+      // Poll each pending item individually instead of reloading everything
+      for (const id of pendingIds) {
+        try {
+          const updated = await api(`/items/${id}`);
+          const idx = items.findIndex(i => i.id === id);
+          if (idx !== -1) {
+            items[idx] = updated;
+            items = items; // trigger reactivity
+          }
+          if (selectedItem?.id === id) {
+            selectedItem = updated;
+          }
+        } catch { /* item might be gone */ }
+      }
+    }, 4000);
   }
 
   function stopPolling() {
     if (pollTimer) { clearInterval(pollTimer); pollTimer = null; }
   }
 
-  // Start polling after capture
+  // Start polling when pending items exist
   $effect(() => {
     const hasPending = items.some(i => i.processing_status === 'pending' || i.processing_status === 'processing');
     if (hasPending && !pollTimer) startPolling();