feat: add automated content moderation module (WIP)

[benedict102]

• Add moderation enums: ContentType, ModerationStatus, ViolationCategory, Severity, ActionType, AppealStatus, QueuePriority, AuditAction
• Add TypeORM entities: ModerationFlag, ModerationAppeal, ModerationAction, ModerationAudit, ModerationQueueItem
• Add DTOs: FlagContentDto, ReviewFlagDto, SubmitAppealDto, ResolveAppealDto, ModerationFilterDto, AnalyzeContentDto
• Add TextFilterService: profanity/pattern matching with sanitization
• Add MLClassificationService: toxicity scoring and category detection
• Add ModerationAuditService: full audit trail logging

What does this PR do?

Why?

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing behavior to change)
• Refactor (no behavior change)
• Docs / CI / tooling

How was it tested?

• Unit tests added/updated
• E2E test added/updated
• Manual verification steps below

Checklist

• npm run lint passes
• npm run format:check passes (run npm run format if not)
• npm run typecheck passes
• npm test passes
• I added/updated tests for the change
• I updated relevant docs (README, JSDoc, comments)
• I added a changeset / migration if schema changed
• I read CONTRIBUTING.md

Screenshots / Logs

Risk & rollout

closes #422

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