feat: add Review entity with session-based constraints

[mijinummi]

⭐ Implement Review Entity & Validation Rules

Context

To support structured feedback and mentor quality tracking, the platform requires a review system tightly coupled to completed mentoring sessions. Reviews must follow strict eligibility rules to preserve data integrity and prevent abuse.

This PR introduces a fully validated Review domain with API-level and database-level enforcement.

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✨ What’s Included

🧱 Review Entity & Schema

• New Review entity linked to:
  • Reviewer (mentee)
  • Reviewee (mentor)
  • Session
• Rating constrained to 1–5
• Optional textual comment
• Database uniqueness enforced on (session, reviewer)
• Indexed for efficient lookups

🔐 Business Rules Enforcement

• Reviews allowed only after session completion
• Reviewer must be the session mentee
• Reviewee must be the session mentor
• Duplicate reviews per session are prevented
• All invalid attempts return clear, meaningful errors

🌐 API

• POST /reviews endpoint to create a review
• Authenticated and rule-protected
• Reviewer identity derived from session context (not user input)

📊 Rating Aggregation (Optional Enhancement)

• Mentor’s average rating recalculated after each new review
• Lays groundwork for analytics, ranking, and reputation features

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🧪 Test Coverage

• Prevents review before session completion
• Blocks non-mentee review attempts
• Prevents duplicate reviews per session
• Allows valid review for completed sessions
• Ensures correct mentor assignment as reviewee

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✅ Acceptance Criteria Checklist

• Review entity and database schema implemented
• API endpoint to create review
• Reviews blocked before session completion
• Reviewer verified as session mentee
• Reviewee verified as session mentor
• Duplicate reviews prevented (DB + service)
• Proper error messages returned
• Unit tests added for entity and rules
• Optional mentor rating aggregation implemented

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🔮 Future Extensions

This implementation supports future enhancements without refactor:

• Review moderation
• Review editing windows
• Mentor responses
• Advanced rating analytics

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Notes for Reviewers

• All rules are enforced at the service layer and backed by DB constraints
• No breaking changes to existing session or user flows
• Designed to scale with analytics and reputation systems
  Closes Review Entity & Rules #150