Harden node lifecycle and API handling

[Rerowros]

Summary

• fail closed when API_KEY is missing, malformed, or the zero UUID
• make backend stop/replacement serializable and prevent a stale keepalive worker from stopping a replacement backend
• return real gRPC empty responses and propagate streaming receive failures
• surface failed Xray runtime user removal instead of falsely confirming revocation
• set HTTP deadlines/body limits and validate WireGuard ticker intervals

Validation

• go test ./controller — passed
• go vet ./...
• git diff --check

Known test-environment limitation

Existing REST/gRPC integration suites require TLS fixtures absent from this checkout; their packages compile before TestMain exits.

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