Improve invoice sending by proxying through backend

Replaces direct webhook calls for invoice sending with a backend API endpoint, utilizing multer for file uploads and form-data for robust data transmission. Includes improved error handling and logging.

Replit-Commit-Author: Agent
Replit-Commit-Session-Id: 1e4433b5-47ab-464c-b663-fea2e53367dd
Replit-Commit-Checkpoint-Type: full_checkpoint
Replit-Commit-Event-Id: e89da884-7d51-4b3d-8af5-22493ce6f2c1
Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/1957c339-2757-4d1f-8e92-e9f71a1ce58e/1e4433b5-47ab-464c-b663-fea2e53367dd/qOJ89TE

package.json

@@ -45,6 +45,7 @@
     "@types/bcrypt": "^6.0.0",
     "@types/form-data": "^2.2.1",
     "@types/memoizee": "^0.4.12",
+    "@types/multer": "^2.0.0",
     "@types/node-fetch": "^2.6.13",
     "@types/pg": "^8.15.4",
     "bcrypt": "^6.0.0",
@@ -69,6 +70,7 @@
     "lucide-react": "^0.453.0",
     "memoizee": "^0.4.17",
     "memorystore": "^1.6.7",
+    "multer": "^2.0.2",
     "nanoid": "^5.1.5",
     "next-themes": "^0.4.6",
     "node-cron": "^4.2.1",