feat: auth middleware, brain data layer, Valkey queue (P1-002, P1-003, P1-004)

Auth middleware (P1-002):
- DatabaseModule provides Db instance with graceful shutdown
- AuthModule mounts BetterAuth at /api/auth/* via toNodeHandler
- AuthGuard validates sessions via BetterAuth API
- CurrentUser decorator extracts user from request

Brain data layer (P1-003):
- CRUD repositories for projects, missions, tasks, conversations
- createBrain(db) factory returns all repositories
- Re-exports drizzle-orm query helpers from @mosaic/db to avoid
  duplicate package resolution

Queue (P1-004):
- ioredis-based Valkey client with createQueue/createQueueClient
- Enqueue/dequeue, pub/sub, queue length operations

Closes #11, Closes #12, Closes #13

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

apps/gateway/package.json

@@ -13,6 +13,8 @@
   },
   "dependencies": {
     "@mariozechner/pi-coding-agent": "~0.57.1",
+    "@mosaic/auth": "workspace:^",
+    "@mosaic/db": "workspace:^",
     "@nestjs/common": "^11.0.0",
     "@nestjs/core": "^11.0.0",
     "@nestjs/platform-fastify": "^11.0.0",
@@ -25,6 +27,7 @@
     "@opentelemetry/sdk-metrics": "^2.6.0",
     "@opentelemetry/sdk-node": "^0.213.0",
     "@opentelemetry/semantic-conventions": "^1.40.0",
+    "better-auth": "^1.5.5",
     "fastify": "^5.0.0",
     "reflect-metadata": "^0.2.0",
     "rxjs": "^7.8.0",