Add error handling to API for better debugging on Plesk

- writeData now returns JSON error if file write fails
- Global error/exception handler returns JSON instead of blank 500

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

api.php

@@ -5,6 +5,14 @@
  */
 session_start();
 header('Content-Type: application/json; charset=UTF-8');
+set_error_handler(function($severity, $message, $file, $line) {
+    throw new ErrorException($message, 0, $severity, $file, $line);
+});
+set_exception_handler(function($e) {
+    http_response_code(500);
+    echo json_encode(['error' => $e->getMessage(), 'file' => basename($e->getFile()), 'line' => $e->getLine()]);
+    exit;
+});
 
 // ─── VAIHDA TÄMÄ SALASANA! ──────────────────────────────────────
 define('ADMIN_PASSWORD', 'passus');
@@ -31,10 +39,18 @@ function readData(string $file, $default = []) {
 }
 
 function writeData(string $file, $data): void {
-    file_put_contents(
-        DATA_DIR . $file,
+    $path = DATA_DIR . $file;
+    $dir  = dirname($path);
+    if (!is_dir($dir)) @mkdir($dir, 0755, true);
+    $result = @file_put_contents(
+        $path,
         json_encode($data, JSON_UNESCAPED_UNICODE | JSON_UNESCAPED_SLASHES | JSON_PRETTY_PRINT)
     );
+    if ($result === false) {
+        http_response_code(500);
+        echo json_encode(['error' => "Tiedoston kirjoitus epäonnistui: $file (kansio: " . (is_writable($dir) ? 'kirjoitettava' : 'EI kirjoitettava') . ")"]);
+        exit;
+    }
 }
 
 function ok($data = []): void {