fix: use runtime BACKEND_URL for proxying (#411)

* fix: use runtime BACKEND_URL for proxying

Fixes #400

* chore: run formatter

ui/src/app/api/v1/[...path]/route.ts

@@ -0,0 +1,104 @@
+import { NextRequest, NextResponse } from "next/server";
+
+import { getServerBackendUrl } from "@/lib/apiClient";
+
+export const dynamic = "force-dynamic";
+export const runtime = "nodejs";
+
+const HOP_BY_HOP_HEADERS = [
+  "connection",
+  "keep-alive",
+  "proxy-authenticate",
+  "proxy-authorization",
+  "te",
+  "trailer",
+  "transfer-encoding",
+  "upgrade",
+];
+
+function trimTrailingSlash(url: string) {
+  return url.endsWith("/") ? url.slice(0, -1) : url;
+}
+
+function buildBackendUrl(request: NextRequest) {
+  const backendUrl = trimTrailingSlash(getServerBackendUrl());
+  return `${backendUrl}${request.nextUrl.pathname}${request.nextUrl.search}`;
+}
+
+function createRequestHeaders(request: NextRequest) {
+  const headers = new Headers(request.headers);
+
+  for (const header of HOP_BY_HOP_HEADERS) {
+    headers.delete(header);
+  }
+
+  headers.delete("accept-encoding");
+  headers.delete("content-length");
+  headers.delete("host");
+
+  return headers;
+}
+
+function createResponseHeaders(response: Response) {
+  const headers = new Headers(response.headers);
+  const setCookies = response.headers.getSetCookie();
+
+  for (const header of HOP_BY_HOP_HEADERS) {
+    headers.delete(header);
+  }
+
+  headers.delete("content-encoding");
+  headers.delete("content-length");
+  headers.delete("set-cookie");
+
+  for (const cookie of setCookies) {
+    headers.append("set-cookie", cookie);
+  }
+
+  return headers;
+}
+
+async function getRequestBody(request: NextRequest) {
+  if (request.method === "GET" || request.method === "HEAD") {
+    return undefined;
+  }
+
+  return request.arrayBuffer();
+}
+
+async function proxyRequest(request: NextRequest) {
+  const backendUrl = buildBackendUrl(request);
+
+  try {
+    const response = await fetch(backendUrl, {
+      method: request.method,
+      headers: createRequestHeaders(request),
+      body: await getRequestBody(request),
+      cache: "no-store",
+    });
+
+    return new Response(request.method === "HEAD" ? null : response.body, {
+      status: response.status,
+      statusText: response.statusText,
+      headers: createResponseHeaders(response),
+    });
+  } catch (error) {
+    const message =
+      error instanceof Error ? error.message : "Unknown backend proxy error";
+
+    return NextResponse.json(
+      {
+        detail: `Backend request failed while proxying to ${backendUrl}: ${message}`,
+      },
+      { status: 502 },
+    );
+  }
+}
+
+export const GET = proxyRequest;
+export const POST = proxyRequest;
+export const PUT = proxyRequest;
+export const PATCH = proxyRequest;
+export const DELETE = proxyRequest;
+export const OPTIONS = proxyRequest;
+export const HEAD = proxyRequest;