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>

gateway/dashboard.py

@@ -188,7 +188,7 @@ def handle_dashboard(handler, user):
     conn.close()
 
     # Services that use gateway-injected API keys (not per-user tokens)
-    GATEWAY_KEY_SERVICES = {"inventory", "reader", "books", "music", "budget", "tasks"}
+    GATEWAY_KEY_SERVICES = {"inventory", "reader", "books", "music", "budget", "tasks", "brain"}
     widgets = []
     futures = {}