security: address audit findings C-4 through M-8

Critical:
- C-4: Add express-rate-limit to login (20 attempts/15min)
- C-5: Remove default credentials from LoginForm.js
- C-6: Add sandbox attribute to KB document iframe

High:
- H-2: Hard-fail on startup if SESSION_SECRET env var is missing
- H-6: Sanitize filenames in Content-Disposition headers
- H-7: Fix KB upload race condition — move file after DB insert succeeds
- H-8: Generate random admin password in setup.js instead of hardcoded
- H-9: Add rehype-sanitize to ReactMarkdown (requires npm install)

Medium:
- M-4: Fix loose equality (==) to strict (===) in users.js self-checks
- M-5: Add hostname format regex validation in compliance notes
- M-6: Fix vendor trim-before-validate in ivantiTodoQueue.js
- M-7: Sanitize original filename in compliance temp JSON
- M-8: Pull CSP frame-ancestors from CORS_ORIGINS env var

New dependencies needed:
- backend: express-rate-limit (npm install in root)
- frontend: rehype-sanitize (npm install in frontend/)

package.json

@@ -15,6 +15,7 @@
     "cors": "^2.8.6",
     "dotenv": "^16.6.1",
     "express": "^5.2.1",
+    "express-rate-limit": "^7.5.0",
     "multer": "^2.0.2",
     "sqlite3": "^5.1.7"
   }