feat: Introduce a web frontend with new city API routes and refactor bot city query logic into a dedicated service.

frontend/lib/backend-auth.ts

@@ -0,0 +1,14 @@
+export const BACKEND_ENTITLEMENT_HEADER = "x-polyweather-entitlement";
+
+export function buildBackendRequestHeaders(): HeadersInit {
+  const headers: HeadersInit = {
+    Accept: "application/json",
+  };
+
+  const token = process.env.POLYWEATHER_BACKEND_ENTITLEMENT_TOKEN?.trim();
+  if (token) {
+    headers[BACKEND_ENTITLEMENT_HEADER] = token;
+  }
+
+  return headers;
+}