Add Soroban contract event indexer and on-chain history source metadata

[anoncon]

Summary

• Emit structured contract events for create_portfolio, deposit, and execute_rebalance.
• Add Soroban contract event indexer service to ingest and persist on-chain events.
• Extend rebalance history storage with on-chain metadata and source flags.
• Expose filtered history by portfolioId, timestamp range, and source.
• Add indexer status and manual sync endpoint.
• Update UI history badges to distinguish On-chain vs Simulated vs Off-chain entries.

Validation

• Backend TypeScript compile passed.
• Queue tests passed.
• Database tests added for on-chain metadata/filtering/dedup; full run blocked locally by native better-sqlite3 binary mismatch.

Closes #43

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