fix(server/proxy): ajout duplex half pour forwarder les POST

- Route proxy: ajout de duplex: 'half' lors de l'envoi d'un body,
  requis par Node.js fetch pour éviter l'erreur 500 sur wp-login.php

server/app/api/proxy/[[...path]]/route.ts

@@ -42,13 +42,13 @@ async function proxyRequest(req: NextRequest) {
   headers.set("x-forwarded-port", "443");
   headers.set("x-forwarded-for", req.headers.get("x-forwarded-for") || "unknown");
 
+  const needsBody = req.method !== "GET" && req.method !== "HEAD";
   const upstreamRes = await fetch(upstream, {
     method: req.method,
     headers,
-    body:
-      req.method !== "GET" && req.method !== "HEAD"
-        ? (req.body as BodyInit)
-        : undefined,
+    body: needsBody ? (req.body as BodyInit) : undefined,
+    // Node.js fetch requires duplex when forwarding a request body stream
+    ...(needsBody ? { duplex: "half" } : {}),
     redirect: "manual",
   });