feat(indexer): scheduled event retention cleanup job for transaction audit log (closes #174)

[s6pa1rta3n-lab]

Closes #174

Summary of Changes

• Added @nestjs/schedule to manage background recurring tasks.
• Imported ScheduleModule.forRoot() in AppModule.
• Created EventRetentionService in server/src/indexer/retention/event-retention.service.ts and registered it in IndexerModule.
• Implemented @Cron(CronExpression.EVERY_DAY_AT_MIDNIGHT) scheduled cleanup job that deletes generic Transaction audit log rows older than soroban.eventRetentionDays (configured via EVENT_RETENTION_DAYS, default 7).
• Deletions are processed in batches (default: 500) to prevent table locks and memory overhead on large tables.
• Audit-Only Scope Decision: Retention is strictly scoped to the generic Transaction audit log. Domain records (Investment, Tranche, Dispute, Order, Campaign, User) are permanently preserved to retain complete financial history and ledger auditability.
• Emits observable logs and metrics (deletedCount, durationMs, batches, cutoffDate, retentionDays) on each cleanup run.
• Documented EVENT_RETENTION_DAYS behavior in .env.example.
• Added unit test suite in server/src/indexer/retention/event-retention.service.spec.ts covering cutoff calculation, batching, zero expired rows, domain data preservation, metric logging, and error handling.
• Added E2E integration test in server/test/event-retention.e2e-spec.ts verifying that expired Transaction rows are pruned while recent transactions and domain rows (Investment, Tranche, Dispute, Campaign, User) remain intact.

Acceptance Criteria Checklist

• A scheduled cleanup job exists, runs on the configured cadence, and respects EVENT_RETENTION_DAYS.
• The job is scoped to audit-only data with explicit documentation preserving domain rows.
• Covered by tests exercising both "older than retention" and "within retention" cases as well as domain entity preservation.

Payout Routing

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