Add security hardening: helmet, CORS allowlist, body limit, ID validation

- Install and configure helmet with basic CSP in app.js
- Restrict CORS to ALLOWED_ORIGIN env var (default localhost:5173)
- Add express.json 1mb body size limit to prevent memory exhaustion
- Add parseInt+isNaN validation for all :id route params in bills.js
  and financing.js (GET/PUT/DELETE/:id and PATCH financing-payments/:id)
- Extend bills.routes.test.js and financing.routes.test.js with ID
  validation tests (non-numeric IDs → HTTP 400)

Nightshift-Task: security-footgun
Nightshift-Ref: https://github.com/marcus/nightshift

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

server/package-lock.json

@@ -11,6 +11,7 @@
         "cors": "^2.8.5",
         "dotenv": "^16.4.5",
         "express": "^4.19.2",
+        "helmet": "^8.1.0",
         "pg": "^8.11.5"
       },
       "devDependencies": {
@@ -1282,6 +1283,15 @@
         "node": ">= 0.4"
       }
     },
+    "node_modules/helmet": {
+      "version": "8.1.0",
+      "resolved": "https://registry.npmjs.org/helmet/-/helmet-8.1.0.tgz",
+      "integrity": "sha512-jOiHyAZsmnr8LqoPGmCjYAaiuWwjAPLgY8ZX2XrmHawt99/u1y6RgrZMTeoPfpUbV96HOalYgz1qzkRbw54Pmg==",
+      "license": "MIT",
+      "engines": {
+        "node": ">=18.0.0"
+      }
+    },
     "node_modules/http-errors": {
       "version": "2.0.1",
       "resolved": "https://registry.npmjs.org/http-errors/-/http-errors-2.0.1.tgz",