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/alembic/env.py

@@ -8,7 +8,7 @@ import os
 sys.path.insert(0, os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 from app.models.base import Base
-from app.models import Category, Question, GameSession, GameEvent, Admin
+from app.models import Category, Question, GameSession, GameEvent, Admin, Achievement
 
 config = context.config
 if config.config_file_name is not None: