fix: harden panel self-upgrade

[Sagit-chu]

This fixes panel self-upgrade and monitor realtime access for permitted non-admin users.

Changes:

• Reuse GitHub proxy config for system upgrade release/API fetches.
• Make panel self-upgrade helper fail fast on compose command errors.
• Allow users with explicit monitor permission to connect to the realtime websocket and receive broadcasts, not just admins.

Validation:

• cd go-backend && GOCACHE=/private/tmp/flvx-gocache go test ./internal/http/handler -count=1
• cd go-backend && go test ./internal/ws -count=1