fix: Fix TypeScript error in PDF generation service cache cleanup

backend/src/services/pdfGenerationService.ts

@@ -197,7 +197,9 @@ class PDFGenerationService {
     // Limit cache size
     if (this.cache.size > 100) {
       const oldestKey = this.cache.keys().next().value;
-      this.cache.delete(oldestKey);
+      if (oldestKey) {
+        this.cache.delete(oldestKey);
+      }
     }
     this.cache.set(cacheKey, { buffer, timestamp: Date.now() });
   }