Require operator token for gRPC pause and resume

[homerquan]

Motivation

• The PauseJob and ResumeJob gRPC RPCs previously allowed unauthenticated clients to change job lifecycle state, creating an availability/control vulnerability.
• The change adds a minimal operator-auth guard to ensure only callers possessing an operator token can invoke these state-changing RPCs.

Description

• Add MirrorNeuron.Grpc.Auth which reads MN_GRPC_OPERATOR_TOKEN and provides authorize_operator!/1 and authorized?/2 to validate bearer or MirrorNeuron-specific token headers with a constant-time compare.
• Gate PauseJob and ResumeJob handlers by calling MirrorNeuron.Grpc.Auth.authorize_operator!(stream) before invoking MirrorNeuron.pause/1 or MirrorNeuron.resume/1.
• Update README.md to document the new MN_GRPC_OPERATOR_TOKEN configuration key.
• Add focused unit tests in tests/unit/grpc/auth_test.exs that exercise header extraction (including adapter/http request headers) and token matching behavior.

Testing

• Ran mix format --check-formatted, which succeeded.
• Ran the new tests directly with elixir -r lib/mirror_neuron_grpc/auth.ex -e 'ExUnit.start()' tests/unit/grpc/auth_test.exs, and they completed with 0 failures.
• mix test could not be run to completion in this environment because Hex/dependency fetches failed (network/proxy prevented mix local.hex / dependency installation).

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Codex Task