feat(indexer): decode Soroban event XDR and upsert contributions & loans (closes #2)

[Vyacheslav-Tomashevskiy]

Closes #2.

Problem

src/common/stellar-indexer.service.ts polls Horizon for contract events, but fetchAndStoreEvents stopped at a // TODO: Parse and upsert… — so the whole sync loop was a no-op and the DB was never populated from on-chain data.

What this does

Implements the decode + upsert step inside the existing structure (keeps the @Cron pollers and the (groupId, contractId, eventType) signature intact — no rewrite of the polling design):

1. Decode each event's topic/value XDR with scValToNative(xdr.ScVal.fromXDR(base64, "base64")). Routing is driven by the event name in the first topic, so it's robust regardless of the poller's eventType hint.
2. Contribution — (member: Address, amount: i128, period: u32) → Contribution, idempotent via upsert on the unique txHash.
3. Loan — loan_requested / loan_approved / loan_repaid (id, borrower, amount[, status]) → Loan, matched by (groupId, onChainId) so lifecycle events update the same row instead of duplicating it (approvedAt / repaidAt set accordingly).
4. Skips events where in_successful_contract_call === false; a single unparseable record is logged and skipped instead of aborting the batch.

The event shapes match exactly what the contracts emit (treasury::contribute publishes ("contribution", (member, amount, period)); loan::request/approve/repay publish (id, borrower, amount[, status])).

Tests

Adds a ts-jest config (ts-jest was already a devDependency but no jest config existed) and stellar-indexer.service.spec.ts, which builds real event XDR with nativeToScVal the same way the contracts emit it and asserts the decode + upsert routing:

PASS src/common/stellar-indexer.service.spec.ts
  ✓ decodes a contribution event and upserts it on txHash
  ✓ decodes a loan_requested event and creates a Pending loan
  ✓ updates an existing loan instead of duplicating it
  ✓ ignores unrelated events (e.g. withdrawal)

No new runtime dependencies (@stellar/stellar-sdk was already present); package-lock.json is untouched.