Fix/indexer cursor

[sebas11042]

Summary

Fixes the Soroban eventIndexer cursor so it no longer derives its watermark from MAX(ledger_sequence), follows RPC pagination fully, and only advances after a clean, fully committed range.

Type of change

• Bug fix
• New feature
• Documentation update
• Refactor / chore
• Smart contract change

Related issue

Closes #910

Changes

• Added an indexer_state migration with last_processed_ledger and last_processed_tx_id.
• Replaced MAX(ledger_sequence) cursor loading with dedicated indexer_state persistence.
• Updated getEvents to follow Soroban RPC pagination cursors until exhausted.
• Made event batch storage transactional and count inserted, conflicts, and errors.
• Advanced the cursor only after conflict-free, error-free ranges.
• Stored RPC event id/txHash in parsed payloads for safer deduplication and cursor metadata.
• Added focused regression tests for pagination, conflicts, insert rollback, empty paged ranges, and cursor persistence.

Testing

• Tested locally on Testnet
• Added/updated unit tests
• Manually tested UI flow

Commands run:

npm.cmd exec -- eslint "src/**/*.js"
npm.cmd test -- eventIndexerCursor.test.js --runInBand
npm.cmd test -- integration-eventIndexer.test.js --runInBand
npm.cmd test -- eventIndexerCursor.test.js integration-eventIndexer.test.js --runInBand 

Checklist

• My code follows the project style
• I've updated docs if needed
• No console errors or warnings
• I've rebased on latest main

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