fix(archiver): shut down API server on stop

[Gercekkriptocu]

Summary

Ensures the Archiver API HTTP server is shut down when the archiver service stops.

Changes

• Stores the Archiver API server returned by httputil.StartHTTPServer
• Shuts it down in ArchiverService.Stop()
• Aligns the archiver service lifecycle behavior with the standalone API service

Why

The archiver service currently starts an API server but does not retain a reference to it, so Stop() cannot shut it down.

Fixes #55

[cb-heimdall]

🟡 Heimdall Review Status

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[Gercekkriptocu]

This PR stores the Archiver API server reference and shuts it down during Stop(), matching the lifecycle behavior used by the standalone API service.