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/server.py

@@ -303,6 +303,11 @@ class GatewayHandler(ResponseMixin, BaseHTTPRequestHandler):
             if user:
                 headers["X-Gateway-User-Id"] = str(user["id"])
                 headers["X-Gateway-User-Name"] = user.get("display_name", user.get("username", ""))
+        elif service_id == "brain":
+            # Inject user identity for the brain service
+            if user:
+                headers["X-Gateway-User-Id"] = str(user["id"])
+                headers["X-Gateway-User-Name"] = user.get("display_name", user.get("username", ""))
         elif user:
             svc_token = get_service_token(user["id"], service_id)
             if svc_token: