Add admin server tests and adapter interfaces

[fabricekabongo]

Summary

• add cluster and broadcast interfaces plus adapter to make the admin server testable
• add unit tests covering admin data and UI handlers
• update main to construct the admin server with the new adapter

Testing

• go test ./...

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Codex Task

[deepsource-io]

Here's the code health analysis summary for commits a8cfbf4..5f0d2fa. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Docker	✅ Success		View Check ↗
Secrets	✅ Success		View Check ↗
Test coverage	❌ Failure	❗ 14 occurences introduced	View Check ↗
Go	❌ Failure	❗ 4 occurences introduced	View Check ↗

Code Coverage Report

Metric	Aggregate	Go
Composite Coverage	63.7% (up 3% from main)	63.7% (up 3% from main)
Line Coverage	63.7% (up 3% from main)	63.7% (up 3% from main)
New Composite Coverage	9.1%	9.1%
New Line Coverage	9.1%	9.1%

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