[715] Backend: Add controlled backfill API for missing transaction index ranges

[MaimunaY]

Summary

• add controlled admin backfill APIs for missing ledger index ranges with idempotent resume/progress reporting
• enforce deterministic secondary sort policy across paginated list responses
• add adaptive per-IP throttle escalation for repeated 4xx abuse patterns
• add immutable export manifest records for generated reporting exports

Validation

• npm run build (pass)
• npm test (blocked locally by corrupted transitive package artifact: bs-logger missing runtime files)

Closes #715
Closes #716
Closes #717
Closes #718

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