implement loan default detection background job

[khaylebfortune]

Summary

Implements the missing default detection background job that automatically detects loans past their grace period and marks them as defaulted.

Changes

New Files

• src/jobs/default-detection.processor.ts — Cron job (@Cron('0 * * * *'), every hour) that:
  • Queries Supabase for active loans where next_payment_due + 7 days < now()
  • Builds a mark_defaulted(loan_id) XDR via CreditLineContractClient.buildMarkDefaultedTx()
  • Signs the XDR with STELLAR_ADMIN_SECRET (server-side keypair)
  • Submits via TransactionsService.submitTransaction()
  • Updates loan status to defaulted with defaulted_at timestamp
  • Clears borrower's reputation_cache so next read picks up on-chain score
  • Falls back to off-chain-only marking if no admin keypair is configured
• src/jobs/jobs.module.ts — Module registering the processor with its StellarModule and TransactionsModule dependencies

Modified Files

• src/stellar/contracts/clients/creditline.client.ts — Added buildMarkDefaultedTx(loanId) that builds an unsigned Soroban XDR calling mark_defaulted on the creditline contract (follows buildRepayLoanTx pattern)
• src/stellar/contracts/interfaces/creditline.interface.ts — Added buildMarkDefaultedTx to ICreditLineClient interface
• src/modules/transactions/dto/submit-transaction-request.dto.ts — Added LOAN_DEFAULT = 'loan_default' to TransactionType enum
• src/app.module.ts — Imported JobsModule
• .env.example — Added STELLAR_ADMIN_SECRET env var documentation

Acceptance Criteria

• Job runs every hour (@Cron('0 * * * *'))
• Detects loans past grace period (7 days from next_payment_due)
• Calls mark_defaulted on-chain via prepared XDR
• Updates local DB status to defaulted
• Decreases borrower reputation (invalidates reputation cache)
• Build passes (verified)
• All 288 tests pass

Verification

• npm run build — passes
• npm run test — 24 suites, 288 tests, all passing

Closes #71