feat(BillingService): implement fallback file path resolution for migrated workspaces

- Added logic to search for PDF templates in known upload directories when the base file name is provided.
- Enhanced the robustness of file handling by checking multiple potential template directories for existing files.
- Improved error handling by returning null for invalid base names, ensuring cleaner path resolution.

backend/services/billingService.js

@@ -51,6 +51,26 @@ class BillingService {
         return candidate;
       }
     }
+
+    // Fallback fuer migrierte/verschobene Workspaces:
+    // Datei ueber den Dateinamen in bekannten Upload-Ordnern suchen.
+    const baseName = path.basename(rawPath);
+    if (!baseName || baseName === '.' || baseName === '/') {
+      return null;
+    }
+    const templateDirs = [
+      path.resolve(process.cwd(), 'uploads', 'billing-templates'),
+      path.resolve(process.cwd(), 'backend', 'uploads', 'billing-templates'),
+      path.resolve(process.cwd(), '..', 'backend', 'uploads', 'billing-templates'),
+      path.resolve(process.cwd(), '..', 'uploads', 'billing-templates')
+    ];
+    for (const dir of templateDirs) {
+      const candidate = path.join(dir, baseName);
+      if (fs.existsSync(candidate)) {
+        return candidate;
+      }
+    }
+
     return null;
   }