Require admin token for ClearJobs RPC

[homerquan]

Motivation

• The ClearJobs gRPC method exposed a destructive, unauthenticated bulk-delete of terminal job records; this change prevents network-reachable clients from invoking the deletion without an explicit administrative credential.

Description

• Add an admin_token field to the ClearJobsRequest protobuf contract and to the generated Elixir protobuf module so requests can carry a confirmation token (proto/job.proto, lib/mirror_neuron_grpc/job.pb.ex).
• Validate the request token inside the gRPC handler by requiring a matching MIRROR_NEURON_GRPC_ADMIN_TOKEN environment value before calling the destructive MirrorNeuron.Monitor.clear_jobs/0, and return permission_denied for unauthenticated requests (lib/mirror_neuron_grpc/server.ex).
• Add a focused regression unit test that asserts unauthenticated ClearJobs calls are rejected prior to any deletion (tests/unit/grpc_job_server_test.exs).
• Document the administrative token requirement in the API section of README.md.

Testing

• Ran mix format successfully to ensure formatting compliance.
• Added a unit test tests/unit/grpc_job_server_test.exs that asserts unauthenticated ClearJobs requests raise a GRPC.RPCError with the permission message, but running mix test was blocked in this environment due to inability to fetch Hex/dependencies (network/CONNECT tunnel 403), so automated test execution could not be completed here.
• Static inspection and local edits confirm the request field and server-side validation are present and will prevent the prior unauthenticated deletion path when the service is run with the environment token configured.

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Codex Task