feat: add push, fetch, and sync latency metrics

[anandh8x]

Summary

• Add Prometheus histograms for successful push and fetch durations.
• Track the origin ref-update timestamp through both peer transports and the sync queue.
• Record sync lag only after the mirror is registered and the queue item completes.

Validation

• cargo fmt --all -- --check
• cargo check --locked --workspace --all-targets
• cargo clippy --locked -p gitlawb-node --bin gitlawb-node -- -D warnings
• Focused metrics, sync, notification, and migration tests
• cargo test --locked -p gitlawb-node (825 passed; two pre-existing concurrency test failures remain outside this change)

[github-actions]

Thanks for the contribution. A couple of things will help us review this faster:

• Link the issue this addresses (Closes #123). For protocol changes, open an issue first.

See CONTRIBUTING.md. Update the PR and these notes will clear automatically.

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