Observability, alerting, and signer wallet funding

[trietlu]

Description

Structured logs across the anchor lifecycle; alert (or a queryable check + runbook) on FAILED_FINAL and on PENDING/SUBMITTED backlog age; monitor the signer wallet balance per chain and warn on low gas.

Affected areas

backend logging, docs/, optional alerting integration.

Dependencies / order

Depends on #8; can run in parallel with #9-#11.

Acceptance criteria

• FAILED_FINAL and stale-backlog conditions produce an alert or a documented queryable check.
• Signer wallet low-balance is surfaced per chain.
• A re-anchoring / incident runbook exists.

Definition of done

• Implemented on a short-lived feature branch, merged into staging (never main).
• Verified on the staging preview stack.
• Docs updated if infra/env/API behavior changes.