feat(en): Start health checks early into EN lifecycle

[slowli]

What ❔

• Implements shared thread-safe container for health checks, AppHealthCheck.
• Refactors EN initialization to start the healthcheck server early into the node lifecycle.
• Adds a healthcheck for snapshot recovery.
• Uses healthchecks in the snapshot recovery integration test.

Why ❔

This increases EN observability.

Checklist

• [x] PR title corresponds to the body of PR (we generate changelog entries from PRs).
• [x] Tests for the changes have been added / updated.
• [x] Documentation comments have been added / updated.
• [x] Code has been formatted via zk fmt and zk lint.
• [x] Spellcheck has been run via zk spellcheck.
• [x] Linkcheck has been run via zk linkcheck.