feat: force a seller scan over active conversations from the inbox

Adds a "Scan for sellers" button to the conversations header, backed by
POST /api/conversations/seller-scan.

The sweep reuses the existing seller detection rather than inventing a
second one: the configured keyword/domain list, the seller-link funnel
patterns, and the aiReplyScreening confidences all decide the verdict, so
a swept conversation lands in the state the live reply loop would have
given it. Being operator-forced, it deliberately ignores filterSellers
and aiReplyScreening.enabled.

AI is the last resort. Conversations are skipped when already flagged or
killed, settled for free by the deterministic rules, or skipped against a
durable sellerScreenedKey watermark when nothing new has arrived since
the last clean sweep. Only the remainder reaches the model, batched eight
at a time through a new screenConversationsForSellers, so the system
prompt and round trip are paid per batch instead of per conversation. A
classifier outage degrades the sweep instead of failing it.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/dashboard/conversations.test.tsx

@@ -604,4 +604,48 @@ describe('ConversationsPage', () => {
     await user.click(within(dialog).getByRole('button', { name: 'Reveal image' }));
     expect(dialogImage).not.toHaveClass('is-blurred');
   });
+  it('runs a forced seller scan and reports what it cost', async () => {
+    let scanned = 0;
+    installConversationList();
+    server.use(
+      http.post('/api/conversations/seller-scan', () => {
+        scanned += 1;
+        return HttpResponse.json({
+          success: true, scanned: 12, alreadyFlagged: 2, unchanged: 6, deterministic: 1,
+          aiScreened: 3, aiRequests: 1, budgetSkipped: 0, emptySkipped: 0, degraded: false,
+          flagged: [{
+            conversationId: 'room-alice', username: 'alice', source: 'ai',
+            reason: 'AI screening classified this conversation as seller (confidence 0.90)',
+            confidence: 0.9, killed: true,
+          }],
+        });
+      }),
+    );
+    const user = userEvent.setup();
+    renderConversations();
+
+    await user.click(await screen.findByRole('button', { name: 'Scan for sellers' }));
+
+    const summary = await screen.findByRole('status');
+    expect(summary).toHaveTextContent('Scanned 12 conversations');
+    expect(summary).toHaveTextContent('1 flagged as sellers (1 ended)');
+    expect(summary).toHaveTextContent('3 needed AI over 1 request');
+    expect(summary).toHaveTextContent('6 unchanged since the last scan');
+    expect(scanned).toBe(1);
+  });
+
+  it('surfaces a failed seller scan without disturbing the list', async () => {
+    installConversationList();
+    server.use(
+      http.post('/api/conversations/seller-scan', () =>
+        HttpResponse.json({ error: 'Failed to scan conversations for sellers' }, { status: 500 })),
+    );
+    const user = userEvent.setup();
+    renderConversations();
+
+    await user.click(await screen.findByRole('button', { name: 'Scan for sellers' }));
+
+    expect(await screen.findByRole('alert')).toHaveTextContent('Seller scan failed');
+    expect(screen.getByRole('button', { name: /u\/alice/ })).toBeInTheDocument();
+  });
 });