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);
   },

backend/services/billingService.js

@@ -613,8 +613,23 @@ class BillingService {
       return { status: 404, response: { success: false, error: 'Abrechnungslauf nicht gefunden' } };
     }
     await checkAccess(userToken, run.clubId);
+    if (!run.template) {
+      console.warn('[billing.generateRun] Vorlage fehlt für Run', {
+        runId: run.id,
+        clubId: run.clubId,
+        templateId: run.templateId
+      });
+      return { status: 404, response: { success: false, error: 'Vorlage für diesen Abrechnungslauf nicht gefunden. Bitte neuen Abrechnungslauf mit aktueller Vorlage erstellen.' } };
+    }
     const resolvedTemplatePath = this._resolveExistingFilePath(run.template?.pdfStoragePath);
     if (!resolvedTemplatePath) {
+      console.warn('[billing.generateRun] Vorlagen-PDF nicht gefunden', {
+        runId: run.id,
+        clubId: run.clubId,
+        templateId: run.templateId,
+        storedPath: run.template?.pdfStoragePath,
+        cwd: process.cwd()
+      });
       return { status: 404, response: { success: false, error: 'Vorlagen-PDF nicht gefunden' } };
     }
     const computed = await this._collectTrainingSessions(run.clubId, run.periodStart, run.periodEnd);