feat: implement Soroban event XDR decoder in StellarIndexerService

[JHON12091986]

Closes #2
/claim #2

Changes

• Implemented Soroban event XDR decoder using @stellar/stellar-sdk
• Added handlers for contribution, loan_requested, and loan_approved events
• Added lastSyncedLedger tracking to avoid duplicate processing
• Added unit tests for event processing

Testing

• ✅ Contribution events upserted correctly
• ✅ Loan requested events handled
• ✅ Loan approved events update status
• ✅ Duplicate events handled gracefully

Technical Details

• Uses scValToNative from @stellar/stellar-sdk to decode XDR payloads
• Implements lastSyncedLedger map to track processed ledgers
• Handles events with upsert operations to avoid duplicates

---

Ready for review | @coopfinance/maintainers

Payment wallet (USDC on Ethereum): 0x46243c949f30280771c0675Bc8A16155A9B96e51

[JHON12091986]

@Vyacheslav-Tomashevskiy Great work on #18! Your implementation is more complete and includes tests. I'm closing my PR (#14) since yours covers everything needed.

@Mona-i I'm closing #14 in favor of #18, which has a more thorough implementation.