Implement NetworkService and replace the placeholder network map with live peer data

[veryCrunchy]

Summary

Implement NetworkService in the Go API and replace the placeholder network map page with live network and peer data.

Scope

• Implement network stats query endpoints.
• Implement peer list/detail endpoints.
• Use shared DB/runtime state to back the service.
• Replace the static frontend network map page.

Why

The frontend currently shows a placeholder network story even though the Rust service already has significant P2P functionality.

Deliverables

• Go NetworkService
• Peer/network stats read models
• Astro network page using live API data

Acceptance Criteria

• Network page no longer relies on generated placeholder values.
• API surfaces for peer/network views are live.
• UI messaging matches actual P2P readiness.