fix(gateway): CORS, memory userId from session, pgvector auto-init

- Enable CORS with credentials (GATEWAY_CORS_ORIGIN env, defaults to
  http://localhost:3000) so the web dashboard can talk to the gateway
- MemoryController: replace @Query('userId') with @CurrentUser() to
  extract userId from auth session (was passing undefined)
- MemoryController: add missing @Inject(EmbeddingService)
- docker-compose: auto-create pgvector extension via init script
- .env.example: add GATEWAY_CORS_ORIGIN

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.env.example

@@ -18,6 +18,7 @@ BETTER_AUTH_URL=http://localhost:4000
 
 # Gateway
 GATEWAY_PORT=4000
+GATEWAY_CORS_ORIGIN=http://localhost:3000
 
 # Discord Plugin (optional — set DISCORD_BOT_TOKEN to enable)
 # DISCORD_BOT_TOKEN=