[API] Implement Soroban event XDR decoder in StellarIndexerService

[Mona-i]

Problem

src/common/stellar-indexer.service.ts polls Horizon for contract events but the fetchAndStoreEvents method has a critical // TODO and does nothing with the events it receives:

// stellar-indexer.service.ts line ~60
this.logger.debug(Found ${records.length} ${eventType} events for contract ${contractId});
// TODO: Parse and upsert into contributions / loans tables
// This is where you'd decode the XDR event payloads from Soroban

Without this, the database is never populated from on-chain data — the whole sync loop is a no-op.

Task

Implement event decoding and database upsert for contribution and loan events:

1. Decode Soroban event XDR payloads

import { xdr, scValToNative } from "@stellar/stellar-sdk";

// Each event record has a value field with base64 XDR
const decoded = scValToNative(xdr.ScVal.fromXDR(record.value.xdr, "base64"));

2. Handle contribution events

Event payload from contract: (member_address, amount, period)
Upsert into Contribution table:

await this.prisma.contribution.upsert({
where: { txHash: record.transaction_hash },
create: { groupId, memberAddress: member, amount, period, txHash, ledger },
update: {},
});

3. Handle loan_requested events
   Event payload: (loan_id, borrower, amount)
   Upsert into Loan table with status "Pending".

4. Handle loan_approved events
   Update matching loan to status: "Approved", set approvedAt.

5. Track last-synced ledger per contract

Store a lastSyncedLedger per contractId in a simple in-memory Map (or new Prisma model) to avoid re-processing old events on every poll.

Acceptance Criteria

• XDR payloads decoded using @stellar/stellar-sdk
• contribution events upserted into Contribution table
• loan_requested and loan_approved events upserted into Loan table
• Duplicate events handled gracefully (upsert, not insert)
• Last-synced ledger tracked to avoid duplicate processing
• Unit test with mocked Horizon response

[JHON12091986]

/claim #2

I'll implement the Soroban event XDR decoder in StellarIndexerService.
Will handle contract events decoding and integrate with the existing indexer.
PR ready in 24 hours.

[Vyacheslav-Tomashevskiy]

Opened #18 for this. Kept the existing @Cron pollers and the (groupId, contractId, eventType) signature — just filled in the decode+upsert step: scValToNative on each event's topic/value XDR, routed by the event name. Contributions are upserted idempotently on txHash; loans are matched by (groupId, onChainId) so loan_requested/approved/repaid update the same row instead of duplicating it. Unsuccessful calls are skipped, and a single unparseable record is logged rather than aborting the batch.

Added a small ts-jest config (ts-jest was already a devDependency but there was no jest config) plus a spec that builds the event XDR the same way the contracts emit it — all green. No new runtime deps and package-lock.json is untouched. Happy to hydrate the loan interestBps/purpose from a contract read instead of defaults if you'd prefer that.

[JHON12091986]

@Mona-i I see there's a new PR (#18) for this issue. I'll review it and compare with my PR (#14). If it's more complete, I'll close mine and focus on other tasks. If not, I'll update my PR with the missing parts.

Quick question: is there a preferred approach for this implementation?