feat: wire brain service to platform gateway

- Gateway proxies /api/brain/* to brain-api:8200/api/* via pangolin network
- User identity injected via X-Gateway-User-Id header
- Brain app registered in gateway database (sort_order 9)
- Added to GATEWAY_KEY_SERVICES for dashboard integration
- Tested: health, config, list, create all working through gateway

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

frontend-v2/package.json

@@ -25,6 +25,7 @@
 		"@sveltejs/adapter-node": "^5.5.4",
 		"@tailwindcss/vite": "^4.2.2",
 		"clsx": "^2.1.1",
+		"pdfjs-dist": "^5.6.205",
 		"tailwind-merge": "^3.5.0",
 		"tailwindcss": "^4.2.2"
 	}