feat: implement dynamic API proxying to FastAPI backend

- Added a new route handler for dynamic API requests, allowing proxying to the FastAPI backend.
- Removed the previous rewrite configuration in next.config.ts for cleaner integration.
- Updated .env.example to clarify backend URL usage.

surfsense_web/app/api/v1/[...path]/route.ts

@@ -0,0 +1,65 @@
+import type { NextRequest } from "next/server";
+
+export const dynamic = "force-dynamic";
+
+const HOP_BY_HOP_HEADERS = new Set([
+	"connection",
+	"keep-alive",
+	"proxy-authenticate",
+	"proxy-authorization",
+	"te",
+	"trailer",
+	"transfer-encoding",
+	"upgrade",
+]);
+
+function getBackendBaseUrl() {
+	const base = process.env.FASTAPI_BACKEND_INTERNAL_URL || "http://localhost:8000";
+	return base.endsWith("/") ? base.slice(0, -1) : base;
+}
+
+function toUpstreamHeaders(headers: Headers) {
+	const nextHeaders = new Headers(headers);
+	nextHeaders.delete("host");
+	nextHeaders.delete("content-length");
+	return nextHeaders;
+}
+
+function toClientHeaders(headers: Headers) {
+	const nextHeaders = new Headers(headers);
+	for (const header of HOP_BY_HOP_HEADERS) {
+		nextHeaders.delete(header);
+	}
+	return nextHeaders;
+}
+
+async function proxy(
+	request: NextRequest,
+	context: { params: Promise<{ path?: string[] }> }
+) {
+	const params = await context.params;
+	const path = params.path?.join("/") || "";
+	const upstreamUrl = new URL(`${getBackendBaseUrl()}/api/v1/${path}`);
+	upstreamUrl.search = request.nextUrl.search;
+
+	const hasBody = request.method !== "GET" && request.method !== "HEAD";
+
+	const response = await fetch(upstreamUrl, {
+		method: request.method,
+		headers: toUpstreamHeaders(request.headers),
+		body: hasBody ? request.body : undefined,
+		// `duplex: "half"` is required by the Fetch spec when streaming a
+		// ReadableStream as the request body. Avoids buffering uploads in heap.
+		// @ts-expect-error - `duplex` is not yet in lib.dom RequestInit types.
+		duplex: hasBody ? "half" : undefined,
+		redirect: "manual",
+	});
+
+	return new Response(response.body, {
+		status: response.status,
+		statusText: response.statusText,
+		headers: toClientHeaders(response.headers),
+	});
+}
+
+export { proxy as GET, proxy as POST, proxy as PUT, proxy as PATCH, proxy as DELETE, proxy as OPTIONS, proxy as HEAD };