fix(GRO-1215): resolve ESLint error, cursor pagination, and UAT playbook gaps

- Add and() + lt() imports from @groombook/db
- Apply businessId to conversation WHERE clause for cross-tenant isolation
  (GET /portal/conversation: clientId AND businessId both scoped)
- Fix cursor pagination: apply lt(messages.createdAt, cursorMsg.createdAt)
  to the cursor WHERE clause so pages actually paginate
- Add UAT_PLAYBOOK.md §4.9.1 Communication tab test cases:
  TC-APP-4.9.6 message history with conversation
  TC-APP-4.9.7 empty state (no conversation yet)
  TC-APP-4.9.8 composer disabled with tooltip
  TC-APP-4.9.9 cross-tenant isolation

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/routes/portal.ts

@@ -255,7 +255,7 @@ portalRouter.get("/conversation/messages", async (c) => {
           deliveredAt: messages.deliveredAt,
         })
         .from(messages)
-        .where(eq(messages.conversationId, conversation.id))
+        .where(and(eq(messages.conversationId, conversation.id), lt(messages.createdAt, cursorMsg.createdAt)))
         .orderBy(desc(messages.createdAt))
         .limit(limit);
     }