fix(gateway): security hardening — auth guards, ownership checks, validation, rate limiting

apps/gateway/package.json

@@ -8,10 +8,11 @@
     "build": "tsc",
     "dev": "tsx watch src/main.ts",
     "lint": "eslint src",
-    "typecheck": "tsc --noEmit",
+    "typecheck": "tsc --noEmit -p tsconfig.typecheck.json",
     "test": "vitest run --passWithNoTests"
   },
   "dependencies": {
+    "@fastify/helmet": "^13.0.2",
     "@mariozechner/pi-ai": "~0.57.1",
     "@mariozechner/pi-coding-agent": "~0.57.1",
     "@mosaic/auth": "workspace:^",
@@ -23,6 +24,7 @@
     "@nestjs/core": "^11.0.0",
     "@nestjs/platform-fastify": "^11.0.0",
     "@nestjs/platform-socket.io": "^11.0.0",
+    "@nestjs/throttler": "^6.5.0",
     "@nestjs/websockets": "^11.0.0",
     "@opentelemetry/auto-instrumentations-node": "^0.71.0",
     "@opentelemetry/exporter-metrics-otlp-http": "^0.213.0",
@@ -33,6 +35,8 @@
     "@opentelemetry/semantic-conventions": "^1.40.0",
     "@sinclair/typebox": "^0.34.48",
     "better-auth": "^1.5.5",
+    "class-transformer": "^0.5.1",
+    "class-validator": "^0.15.1",
     "fastify": "^5.0.0",
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",