fix: catch Throwable instead of Exception, add prepare() null checks

PHP's PDO in silent mode returns false on prepare() when table doesn't
exist; calling ->execute() on false throws TypeError (Error, not
Exception). Changed catch to \Throwable and added explicit checks so
the real error message is returned as JSON instead of HTTP 500 empty.

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

api/lab/ingest_diagnosticos.php

@@ -43,10 +43,14 @@ try {
          VALUES (?, ?)
          ON DUPLICATE KEY UPDATE concepto = VALUES(concepto)"
     );
+    if (!$stmt) {
+        $info = $pdo->errorInfo();
+        throw new \RuntimeException("Prepare falló [{$info[0]}]: {$info[2]}");
+    }
 
     $insertados = 0;
     foreach ($data['rows'] as $row) {
-        $cod     = trim($row['cod']     ?? '');
+        $cod      = trim($row['cod']      ?? '');
         $concepto = trim($row['concepto'] ?? '');
         if (!$cod || !$concepto) continue;
         $stmt->execute([$cod, $concepto]);
@@ -55,7 +59,7 @@ try {
 
     ob_clean();
     echo json_encode(['ok' => true, 'insertados' => $insertados]);
-} catch (Exception $e) {
+} catch (\Throwable $e) {
     ob_clean();
     http_response_code(500);
     echo json_encode(['ok' => false, 'error' => $e->getMessage()]);