feat(BillingController, BillingService): enhance billing template handling and error logging

- Updated billingController to use a dynamic upload directory for billing templates, improving file management.
- Added error handling in billingService to log warnings when templates are missing or not found, enhancing debugging capabilities.
- Improved user feedback by returning specific error messages when template-related issues occur during billing runs.

backend/controllers/billingController.js

@@ -1,11 +1,16 @@
 import fs from 'fs';
 import multer from 'multer';
 import path from 'path';
+import { fileURLToPath } from 'url';
 import billingService from '../services/billingService.js';
 
+const __filename = fileURLToPath(import.meta.url);
+const __dirname = path.dirname(__filename);
+const BILLING_TEMPLATE_UPLOAD_DIR = path.resolve(__dirname, '..', 'uploads', 'billing-templates');
+
 const storage = multer.diskStorage({
   destination: (req, file, cb) => {
-    const dir = 'uploads/billing-templates';
+    const dir = BILLING_TEMPLATE_UPLOAD_DIR;
     fs.mkdirSync(dir, { recursive: true });
     cb(null, dir);
   },