Fix TypeScript: add @types/ws and RawData type for WebSocket message

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/package-lock.json

@@ -25,6 +25,7 @@
         "@types/node": "^20.14.0",
         "@types/nodemailer": "^7.0.11",
         "@types/pg": "^8.11.6",
+        "@types/ws": "^8.18.1",
         "tsx": "^4.15.7",
         "typescript": "^5.4.5"
       }
@@ -657,6 +658,16 @@
         "pg-types": "^2.2.0"
       }
     },
+    "node_modules/@types/ws": {
+      "version": "8.18.1",
+      "resolved": "https://registry.npmjs.org/@types/ws/-/ws-8.18.1.tgz",
+      "integrity": "sha512-ThVF6DCVhA8kUGy+aazFQ4kXQ7E1Ty7A3ypFOe0IcJV8O/M511G99AW24irKrW56Wt44yG9+ij8FaqoBGkuBXg==",
+      "dev": true,
+      "license": "MIT",
+      "dependencies": {
+        "@types/node": "*"
+      }
+    },
     "node_modules/abstract-logging": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/abstract-logging/-/abstract-logging-2.0.1.tgz",