feat(gRPC): Allow configuring an auth token

[joschahenningsen]

Tested locally like this:

go run cmd/voice-service/main.go -auth-token abc &

✗ grpcurl -plaintext \
  -d '{"language": "en", "stream_id":1, "source":"https://example.com"}' \
  -import-path ./protobufs -proto subtitles.proto \
  localhost:50051 live.voice.v1.SubtitleGenerator.Generate
ERROR:
  Code: Unauthenticated
  Message: auth token is not provided

[Copilot]

Pull request overview

This PR adds authentication token support to the voice service gRPC server and client. The token is configured via a command-line flag and used to authenticate both incoming and outgoing gRPC requests.

• Adds an authToken field to the App struct and updates the constructor to accept it
• Implements a gRPC unary interceptor to validate incoming requests against the configured token
• Appends the auth token to outgoing gRPC metadata when calling the subtitle receiver

Reviewed changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated 3 comments.

File	Description
pkg/voiceservice/voiceservice.go	Adds auth token field, implements authentication interceptor, and attaches token to outgoing requests
cmd/voice-service/main.go	Adds auth-token command-line flag and passes it to the voiceservice constructor

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