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"]

backend/app/models/achievement.py

@@ -0,0 +1,22 @@
+from sqlalchemy import Column, Integer, String, Text, DateTime, ForeignKey, func
+from sqlalchemy.orm import relationship
+from app.models.base import Base
+
+
+class Achievement(Base):
+    __tablename__ = "achievements"
+
+    id = Column(Integer, primary_key=True, index=True)
+    name = Column(String(100), nullable=False, unique=True)
+    description = Column(Text, nullable=False)
+    icon = Column(String(10), nullable=False)
+    condition_type = Column(String(50), nullable=False)
+    condition_value = Column(Integer, default=1)
+    category_id = Column(Integer, ForeignKey("categories.id"), nullable=True)
+    created_at = Column(DateTime, server_default=func.now())
+
+    # Relationships
+    category = relationship("Category")
+
+    def __repr__(self):
+        return f"<Achievement(id={self.id}, name='{self.name}')>"