feat(indexer): add getLagSnapshot endpoint and corresponding service …

[PeterOche]

Summary

Adds an operational monitoring endpoint for indexer lag.

Changes

• Added GET /indexer/lag in backend/src/indexer/indexer.controller.ts
• Added getLagSnapshot() in backend/src/indexer/indexer.service.ts
• Added documented response schema in backend/src/indexer/dto/indexer-lag-response.dto.ts
• Added tests for service lag calculation and endpoint response shape

Response Includes

• network + stream cursor snapshot
• last processed tx hash
• latest network ledger
• derived ledger lag
• replay skip / ingest / error / poll counters

Validation

• Added endpoint integration test for schema + values
• Added service test for lag calculation behavior

Notes

• Did not address unrelated pre-existing backend/test environment issues
• Local test execution is blocked in this workspace because jest is not installed

closes #638

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