[W5][BE][S] Add healthcheck endpoint for indexer queue depth and worker liveness

[kike-alt]

ID: W5-3C-BE-002
Track: BE
Phase: 3C
Difficulty: beginner
Priority: P1

Description

Expose liveness/readiness signals for queue depth and worker heartbeat.

Acceptance Criteria

• Define implementation plan aligned to scope boundaries
• Implement and verify changes in backend/src/indexer/indexer.controller.ts
• Implement and verify changes in backend/src/indexer/queue
• Add/adjust tests or validation artifacts for the changed surface
• Update docs/notes if behavior or operations change

Relevant Files/Modules

• backend/src/indexer/indexer.controller.ts
• backend/src/indexer/queue

Dependencies / Blocked By

• [W5][BE][M] Add indexer lag endpoint for operational monitoring #638

Validation Requirements

• endpoint tests validate schema and status transitions