feat(backend): improve observability and readiness

[ololadedavidvictor-bit]

Summary

• Add Pino JSON logging with request correlation IDs, LOG_LEVEL, secret redaction, and development-only pretty output.
• Expose Prometheus HTTP latency, process, indexer lag, and webhook outcome metrics; support disabling /metrics.
• Add bounded MongoDB, Redis, and Soroban RPC readiness checks while preserving /health as liveness.
• Ensure MongoDB indexes idempotently at connection/startup and document operational endpoints and metrics.

Validation: no compile or tests run, as requested. Static whitespace and logging inventory checks passed.

Closes #696
Closes #697
Closes #698
Closes #699

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