feat(phase2): Add achievements and replay systems

Achievement System:
- Add Achievement model with condition types (streak, steal, specialist, etc.)
- Add AchievementManager service for tracking and awarding achievements
- Add Pydantic schemas for achievements (AchievementResponse, PlayerStats, etc.)
- Seed 18 achievements from design doc
- Add GET /api/game/achievements endpoint

Replay System:
- Add ReplayManager service for saving/loading game replays
- Add GET /api/replay/{code} and /api/replay/session/{id} endpoints
- Format replays for frontend consumption

Phase 2 tasks completed:
- F2.1: Achievement model and migration
- F2.2: Pydantic schemas
- F2.3: AchievementManager service
- F2.4: ReplayManager service
- F2.5: API endpoints
- F2.6: Seed 18 achievements data

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -3,5 +3,6 @@ from app.models.question import Question
 from app.models.game_session import GameSession
 from app.models.game_event import GameEvent
 from app.models.admin import Admin
+from app.models.achievement import Achievement
 
-__all__ = ["Category", "Question", "GameSession", "GameEvent", "Admin"]
+__all__ = ["Category", "Question", "GameSession", "GameEvent", "Admin", "Achievement"]