Add backend package-lock.json for npm ci support

backend/package-lock.json

@@ -0,0 +1,25 @@
+{
+  "name": "png-editor-backend",
+  "version": "0.1.0",
+  "lockfileVersion": 3,
+  "requires": true,
+  "packages": {
+    "": {
+      "name": "png-editor-backend",
+      "version": "0.1.0",
+      "dependencies": {
+        "express": "^4.19.0",
+        "multer": "^1.4.5",
+        "sharp": "^0.33.0",
+        "cors": "^2.8.5"
+      },
+      "devDependencies": {
+        "@types/express": "^4.17.21",
+        "@types/multer": "^1.4.7",
+        "@types/cors": "^2.8.17",
+        "ts-node-dev": "^2.0.0",
+        "typescript": "^5.6.0"
+      }
+    }
+  }
+}