[New feature] Add Badges and Achievement System

[xXPinkmagicXx]

This will most likely make the learning experience nicer. This can work great with #883, #884, #885 and #886.

You could get badges when

• You reach a certain level
• When you learn 10, 25 and 100 words
• When you do 10, 25 and 100 exercises
• For reaching a certain rank (Point system that capture expertise)

[xXPinkmagicXx]

Ideas for Achievements

1. Achievement for reaching a certain level
2. Achievement for activities/exercises
   • For amount of time spent
   • For doing x amount of times
3. Leaderboard badges
   • For ending up in top 5, top 5% ( with a date too)
4. Achievement for consistency ( streaks )
5. Achievement for exploration
   • Try all exercise types
   • Complete first listening exercise
6. Social Achievements (If friendship system is added)
   • Add first friend
   • Send 5 encouragements
7. Hidden achievements
8. Show progress bar

[xXPinkmagicXx]

Database Schema for Achievement System

erDiagram

    ACHIEVEMENTS {
        int id PK
        string name
        string description
        string badge_icon
        string category
        enum metric_type
        int target_value
        datetime created_at
    }

    USER_ACHIEVEMENTS {
        int id PK
        int user_id FK
        int achievement_id FK
        datetime earned_at
        int progress_value
    }

    USER_ACHIEVEMENT_PROGRESS {
        int id PK
        int user_id FK
        int achievement_id FK
        int current_value
        datetime last_updated
    }

    USER_METRICS {
        int user_id FK
        enum metric_type
        int value
    }

    USERS {
        int id PK
    }

    USERS ||--o{ USER_ACHIEVEMENTS : earns
    ACHIEVEMENTS ||--o{ USER_ACHIEVEMENTS : awarded

    USERS ||--o{ USER_ACHIEVEMENT_PROGRESS : tracks
    ACHIEVEMENTS ||--o{ USER_ACHIEVEMENT_PROGRESS : progresses

    USERS ||--o{ USER_METRICS : has

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[xXPinkmagicXx]

For the achievements we should also consider the visibility of achievements/badges. Can friends see all achievements?
Do we want an option for a user to hide all or some achievements? In that case we has to add that to the schema

[gabortodor]

One thing we should keep in mind is that we might not want to store achievement progress, only the milestones (USER_ACHIEVEMENTS) for some types. For certain simpler metrics, just querying the existing data could be faster and better. This way we could also avoid potential inconsistencies