feat: expose farm observability endpoints

[loonghao]

Closes #10

Summary

• add machine-readable health, metrics, and audit models
• expose readiness, metrics, and audit routes from the controller
• record audit events for control-plane mutations and track lease-renewal/error counters
• document endpoint usage in the architecture guide

Verification

• cargo test --workspace
• cargo clippy --workspace --all-targets -- -D warnings
• npm run build