Forward JSON registration POST bodies through the ladl.link proxy.

Event signup from ladl.link sends application/json; re-encoding as form
data dropped the payload and caused failed registrations to redirect to
ladl.link/undefined.

Co-authored-by: Cursor <cursoragent@cursor.com>

app/Services/Link/LinkPlatformProxy.php

@@ -74,7 +74,7 @@ class LinkPlatformProxy
             ->withHeaders($this->forwardHeaders($request));
 
         $pending = match ($method) {
-            'POST' => $client->asForm()->post($target, $request->post()),
+            'POST' => $this->forwardPost($client, $request, $target),
             'PATCH' => $client->patch($target, $request->all()),
             'DELETE' => $client->delete($target),
             default => $client->get($target),
@@ -143,12 +143,27 @@ class LinkPlatformProxy
         return array_filter([
             'Accept' => $request->header('Accept'),
             'Accept-Language' => $request->header('Accept-Language'),
+            'Content-Type' => $request->header('Content-Type'),
             'User-Agent' => $request->header('User-Agent'),
             'Referer' => $request->header('Referer'),
             self::INTERNAL_HEADER => '1',
         ]);
     }
 
+    private function forwardPost(\Illuminate\Http\Client\PendingRequest $client, Request $request, string $target): \Illuminate\Http\Client\Response
+    {
+        if ($this->requestSendsJson($request)) {
+            return $client->asJson()->post($target, $request->all());
+        }
+
+        return $client->asForm()->post($target, $request->post());
+    }
+
+    private function requestSendsJson(Request $request): bool
+    {
+        return str_contains(strtolower((string) $request->header('Content-Type', '')), 'application/json');
+    }
+
     /** @param array<string, array<int, string>> $headers */
     private function sanitizeHeaders(array $headers): array
     {