chore: upgrade express-rate-limit 7.5.1 → 8.3.1 (IPv6 security fix)

- Fixes IPv6 rate limit bypass vulnerability (GHSA-46wh-pxpv-q5gq)
- IPv6 addresses now masked to /56 subnet by default
- Updated custom keyGenerators to use ipKeyGenerator() helper
- 5 new TDD tests for v8 features (ipKeyGenerator, IPv6 masking)
- 672 tests passing, 0 TS errors, 0 npm audit vulnerabilities

package.json

@@ -14,7 +14,7 @@
   "dependencies": {
     "compression": "^1.8.1",
     "express": "^5.1.0",
-    "express-rate-limit": "^7.5.1",
+    "express-rate-limit": "^8.3.1",
     "helmet": "^8.1.0",
     "marked": "^17.0.4",
     "nanoid": "^5.1.6",