Add Achievement System to Progress Tracker

[rhemy-arc]

Here's a comprehensive PR message for the achievement system implementation:

Add Achievement System to Progress Tracker
Summary
Implements a gamification achievement system that tracks learner milestones to drive engagement and motivate continued learning. The system awards achievements for key learning milestones and makes them queryable via on-chain functions.

Changes

1. New Data Structures (types.rs):

Added Achievement struct with fields: achievement_type, earned_at, and optional course_id
Added AchievementType enum with values: FirstCourse, PerfectScore, SpeedLearner, CourseMaster, PerfectCourse
Extended ProgressTrackerDataKey enum with: Achievements(Address) and AchievementEarned(Address, AchievementType)
2. Achievement Functions (lib.rs):

earn_achievement(): Internal function that checks for duplicates before awarding achievements
get_achievements(): Public query function to retrieve all achievements for a learner
has_achievement(): Public query to check if a learner has earned a specific achievement type
3. Achievement Triggers:

FirstCourse: Awarded when eligible_for_credential becomes true (course completion)
PerfectScore: Awarded when submitting a quiz with 100% score
CourseMaster: Awarded when completing 5+ courses (checks via get_learner_stats)
4. Events:

Added achievement_earned event emitted when any achievement is earned
Key Features
✅ Achievements are tracked - Each achievement is stored with timestamp and optional course context ✅ Achievements are queryable - get_achievements() returns chronological list, has_achievement() checks specific types ✅ Achievements emit events - achievement_earned events notify indexers in real-time ✅ Achievement types are extensible - Enum-based design allows easy addition of new achievement types

Technical Details
Achievements are stored in two keys: list of all achievements per learner, and deduplication map
Internal earn_achievement() prevents duplicate awards for same achievement type
Achievements automatically trigger when relevant milestones are reached (no manual claiming needed)
Achievement data is persisted with TTL extensions for long-term storage
Testing Considerations
FirstCourse achievement triggers when credential eligibility is reached
PerfectScore achievement triggers on quiz submissions with score=100
CourseMaster achievement triggers when 5+ courses completed
Events are emitted for indexer consumption
Query functions return empty lists for learners with no achievements
Future Extensions
The system is designed to be easily extended with:

Additional achievement types (e.g., PerfectCourse for 100% completion)
SpeedLearner achievement (requires tracking course completion timestamps)
Achievement badges or visual representations in frontend
Achievement-based rewards or leaderboards

Closes #258
Closes #257
Closes #256
Closes #255

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