Add Gateway proxy router

[fengtality]

Summary

• Adds a catch-all proxy router at /api/gateway-proxy/* that forwards requests to the Gateway server unchanged
• This allows the dashboard to access all Gateway endpoints through the API without needing each endpoint to be explicitly defined
• Protected by HTTP Basic Auth like all other routes

Examples

GET /api/gateway-proxy/wallet -> GET localhost:15888/wallet
POST /api/gateway-proxy/wallet/add -> POST localhost:15888/wallet/add
GET /api/gateway-proxy/config -> GET localhost:15888/config

Security Review

• ✅ Protected by authentication (same auth_user dependency as other routers)
• ✅ Fixed target host (Gateway URL from config, not user-controllable)
• ✅ No SSRF risk - forwards only to internal Gateway service

Test plan

• Verify authentication is required
• Test forwarding GET/POST requests to Gateway
• Test health check endpoint at /api/gateway-proxy

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