feat: add Zero query endpoint and configure ZERO_QUERY_URL

- Create app/api/zero/query/route.ts — resolves named queries to ZQL
  using handleQueryRequest and mustGetQuery from @rocicorp/zero
- Add ZERO_QUERY_URL to both docker-compose files:
  - dev: http://host.docker.internal:3000 (reaches local Next.js from Docker)
  - prod: http://frontend:3000 (Docker service networking)

Without this endpoint, zero-cache cannot resolve named queries and
no data syncs to the client.

surfsense_web/app/api/zero/query/route.ts

@@ -0,0 +1,18 @@
+import { mustGetQuery } from "@rocicorp/zero";
+import { handleQueryRequest } from "@rocicorp/zero/server";
+import { NextResponse } from "next/server";
+import { queries } from "@/zero/queries";
+import { schema } from "@/zero/schema";
+
+export async function POST(request: Request) {
+	const result = await handleQueryRequest(
+		(name, args) => {
+			const query = mustGetQuery(queries, name);
+			return query.fn({ args, ctx: undefined });
+		},
+		schema,
+		request,
+	);
+
+	return NextResponse.json(result);
+}