feat: Add student performance statistics to admin panel

[Nicolapps]

Problem: The admin users page only shows binary exercise completion (✓ or —). Instructors have no visibility into how students engaged — time spent, number of attempts, AI interactions, etc. This makes it hard to identify struggling students or evaluate effort.

Proposed Solution: Add per-student performance metrics to the admin panel, including:

• Total exercises attempted and completed
• Time spent per exercise and total time spent
• Number of quiz attempts per exercise
• Number of prompts sent to the AI assistant
• Time spent on lectures

The data sources already exist (logs, attempts, messages, quizSubmissions, lectureChatMessages tables). This requires new aggregation queries on the backend and a statistics view (or expanded users table) on the frontend.