test+feat: unit tests for audit-log/reporting/grading services + notification-preferences indexes

[bobaivigitalpoint-ui]

Summary

Resolves four assigned issues in a single PR.

Changes

• Add unit tests for audit-log service #1264 — src/audit-log/audit-log.service.spec.ts: unit tests covering all public methods of the AuditLogService facade (log, logAuth, logDataChange, logApiAccess, logSecurityEvent, search, findAll, findByUser, findByAction, findByEntity, findByIpAddress, findByDateRange, generateReport, getStatistics, applyRetentionPolicy, exportToJson, exportToCsv)
• Add unit tests for audit-reporting service #1266 — src/audit-log/services/audit-reporting.service.spec.ts: unit tests for AuditReportingService covering generateReport (zero totals, aggregations, top-users mapping, Unknown email fallback) and getStatistics (parallel counts, correct severity filters)
• Add unit tests for grading service #1261 — src/assessment/grading/grading.service.spec.ts: unit tests for GradingService covering gradeSubmission (score mismatch, unknown criterion, duplicate criterion, missing level/points, point capping, levelId resolution, missing attempt) and autoGradeSubmission (not enabled, missing default level, happy path) and findByAttempt (found, not found)
• Add database indexes to the notification-preferences entity #1248 — @Index decorators added to notification-preferences.entity.ts (userId, globalUnsubscribe) plus migration 1800000000000-add-notification-preferences-indexes.ts to create the indexes on existing databases

CI

• Lint: 0 errors ✅
• TypeScript typecheck: clean ✅
• Build: passes ✅
• migrations:check: all 33 files pass ✅

Closes #1264
Closes #1266
Closes #1261
Closes #1248

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