feat(settings): add audit retention days to general settings API

[hashbk]

Summary

• Add the audit log retention days configuration to the existing GET/PUT /api/settings/general endpoint, replacing the standalone AuditSettingsService that had no API exposure
• Administrators can now configure audit retention alongside other general settings without a separate endpoint

Changes

File	Change
dto/general-settings.dto.ts	Add auditRetentionDays to GeneralSettingsDto and UpdateGeneralSettingsDto with @IsInt and @Min(0)
services/general-settings.service.ts	Add AUDIT_RETENTION_DAYS_KEY, getAuditRetentionDays(), and readAuditRetentionDays()
audit/services/audit-cleanup.service.ts	Inject GeneralSettingsService instead of AuditSettingsService
services/audit-settings.service.ts	Deleted — logic consolidated into GeneralSettingsService
settings.module.ts	Remove AuditSettingsService from providers/exports
general-settings.service.spec.ts	Add tests for persistence, fallback, and DTO validation

Usage

PUT /api/settings/general
{
  "watermarkEnabled": true,
  "auditRetentionDays": 30
}

A value of 0 disables auto-cleanup (default). The daily cron job in AuditCleanupService reads this setting and deletes audit records older than the configured number of days.