Calculate and update user reputation based on contributions.

Acceptance Criteria:

• Create IReputationService interface
• Point values:
  • Comment upvote received: +5
  • Comment downvote received: -2
  • Report approved: +25
  • Vulnerability added (by severity): Critical +50, High +30, Medium +15, Low +5
  • Rating/review posted: +10
• Update reputation on relevant events
• Store current reputation on UserProfile
• Create background job for recalculation (data integrity)
• Add reputation history log