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>

apps/dashboard/src/features/conversations/conversation-queries.ts

@@ -182,6 +182,20 @@ export function useBulkMarkConversationsReadMutation() {
   });
 }
 
+/**
+ * Forces a seller pass over every active conversation. The list is refetched
+ * on completion because a sweep can flag, kill and resolve conversations.
+ */
+export function useSellerScanMutation() {
+  const queryClient = useQueryClient();
+  return useMutation({
+    mutationFn: () => conversationApi.sellerScan(),
+    async onSuccess() {
+      await queryClient.invalidateQueries({ queryKey: conversationQueryKeys.all });
+    },
+  });
+}
+
 export function useStartConversationMutation() {
   const queryClient = useQueryClient();
   return useMutation({