feat: implement ML-assisted content moderation system

[iyanumajekodunmi756]

Implements a comprehensive ML-based content moderation system that automatically pre-screens submissions for policy violations, routes high-risk content to a human review queue, and uses moderator decisions to improve the model over time.

Key features:

• ML pre-screening service with pattern-based policy violation detection across 14 policy categories (hate speech, spam, NSFW, violence, etc.)
• Multi-factor risk scoring: text analysis, metadata, user history, content similarity, and per-policy violation scores
• Auto-approval/rejection for high-confidence decisions with configurable thresholds
• Priority-based human review queue with moderator assignment and load balancing
• Complete appeal flow: rejected content authors can submit appeals with evidence, reviewed by admins
• Model feedback loop: moderator decisions are tracked as correct/incorrect predictions for accuracy monitoring and retraining
• Async background scoring worker for processing batches of submissions
• RESTful API at /api/moderation with full validation (submit, score, queue, decide, appeal)
• Enhanced admin moderation UI with ML risk score visualization, severity indicators, policy breakdown, and dark-themed appeal review
• Comprehensive test suite (23 tests covering scoring, queuing, appeals, integration lifecycle, and performance)

Files modified:

• backend/src/models/Moderation.ts (new: data model/types)
• backend/src/services/moderation/ (new: scoring, queue, appeal services)
• backend/src/workers/moderationJob.ts (new: async scoring worker)
• backend/src/controllers/moderationController.ts (new: REST controller)
• backend/src/routes/moderation.ts (new: API routes with validation)
• backend/src/index.ts (register moderation routes and worker)
• backend/tests/moderation.test.js (new: test suite)
• backend/tests/setup.js (fix: mock roles utility for UserRole export)
• backend/jest.moderation.config.js (new: standalone test config)
• frontend/src/app/admin/content/moderation/page.tsx (enhanced UI)

Closes #402