feat: egress proxy operations, filtering, and batch deletion

[jun9100]

Summary

• add encrypted HTTP(S) proxy subscription sources and plain/Base64 HTTP/SOCKS list import
• add per-node probing, exit-IP health state, scheduled sync/probe jobs, and automatic capacity-aware account allocation
• support explicit batch account-to-proxy bindings, protected from automatic rebalancing
• add account egress-state filtering (bound / unbound) and proxy-node filters for name, scope, enabled state, probe state, and assigned-account state
• add selection-based batch proxy deletion; the relational implementation clears affected account bindings in the same transaction
• add admin UI for proxy operations, node capacity/probing, account binding, filtering, and bulk deletion

Safety

• subscription URLs and proxy credentials remain write-only/encrypted at rest
• subscription fetching rejects non-public destinations
• manual bindings require an enabled configured proxy and never silently fall back
• batch node deletion removes the corresponding account binding metadata atomically

Validation

• go test ./...
• go test -race ./...
• go vet ./...
• pnpm lint
• pnpm build
• deployed-instance smoke test for account egress filters and selection-based batch deletion