fix: Swiss API needs Content-Type + empty body {} for bodiless POSTs

The API rejects bodiless POSTs without Content-Type (415 Unsupported Media
Type) AND rejects Content-Type with literally no body (Body cannot be empty).
Solution: always send Content-Type, and for POST/PATCH/PUT with null body,
send empty object {} as body.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

core/ApiProxy.php

@@ -15,14 +15,15 @@ class ApiProxy
 
     public static function swiss(string $method, string $path, ?array $body = null, ?string $token = null): array
     {
-        $headers = [];
-        if ($body !== null) {
-            $headers[] = 'Content-Type: application/json';
-        }
+        $headers = ['Content-Type: application/json'];
         $authToken = $token ?? self::getSwissToken();
         if ($authToken) {
             $headers[] = 'Authorization: Bearer ' . $authToken;
         }
+        $upperMethod = strtoupper($method);
+        if ($body === null && in_array($upperMethod, ['POST', 'PATCH', 'PUT'])) {
+            $body = [];
+        }
         return self::request($method, SWISS_API_URL . $path, $body, $headers);
     }