feat(20-02): add splitLink to route subscriptions to SSE httpSubscriptionLink

Replace single httpBatchLink with splitLink that routes subscription
operations to httpSubscriptionLink (SSE) while keeping queries/mutations
on httpBatchLink. No new packages needed — both exports are part of
@trpc/client v11.

packages/web/src/lib/trpc.ts

@@ -1,5 +1,5 @@
 import { createTRPCReact } from '@trpc/react-query';
-import { httpBatchLink } from '@trpc/client';
+import { httpBatchLink, splitLink, httpSubscriptionLink } from '@trpc/client';
 import type { AppRouter } from '@codewalk-district/shared';
 
 export const trpc = createTRPCReact<AppRouter>();
@@ -7,10 +7,10 @@ export const trpc = createTRPCReact<AppRouter>();
 export function createTRPCClient() {
   return trpc.createClient({
     links: [
-      httpBatchLink({
-        // In dev, Vite proxy forwards /trpc to backend
-        // In prod, same-origin or configured URL
-        url: '/trpc',
+      splitLink({
+        condition: (op) => op.type === 'subscription',
+        true: httpSubscriptionLink({ url: '/trpc' }),
+        false: httpBatchLink({ url: '/trpc' }),
       }),
     ],
   });