Add structured log for indexer startup showing last processed ledger and sync lag

[Chucks1093]

Summary

When the indexer starts, operators need to know immediately how far behind it is from the current network ledger. A startup log showing the last processed ledger and the lag in ledgers makes it easy to assess sync state without querying the database.

Scope

• On indexer startup, emit a structured info log with fields: last_processed_ledger, current_network_ledger, lag_in_ledgers, estimated_catchup_seconds
• estimated_catchup_seconds calculated as lag_in_ledgers * 5 (5 seconds per ledger)
• If no ledger has been processed yet emit a warning indicating a fresh start

Acceptance Criteria

• Startup log emitted with all four fields
• Fresh start emits a warning instead of info
• estimated_catchup_seconds is correctly calculated from the lag

ETA: 12 hours

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Coordinate on Telegram

[BigJohn-dev]

@BigJohn-dev has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Allow me fix this pls

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[codefather2026]

@codefather2026 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi maintainer, i will like to solve this issue

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[Oluwasuyi-Timilehin]

@Oluwasuyi-Timilehin has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hello, please assign this to me. I would get started immediately

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[parkerwinner]

@parkerwinner has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi, am fimiliar with server issues and can resolve this issue robustly, kindly assign it to me

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