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/schemas/achievement.py

@@ -0,0 +1,50 @@
+from pydantic import BaseModel
+from typing import Optional, List
+from datetime import datetime
+
+
+class AchievementBase(BaseModel):
+    name: str
+    description: str
+    icon: str
+    condition_type: str
+    condition_value: int = 1
+    category_id: Optional[int] = None
+
+
+class AchievementCreate(AchievementBase):
+    pass
+
+
+class AchievementResponse(AchievementBase):
+    id: int
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class PlayerAchievement(BaseModel):
+    """Logro desbloqueado por un jugador (almacenado en localStorage)"""
+    achievement_id: int
+    unlocked_at: datetime
+    game_session_id: Optional[int] = None
+
+
+class AchievementUnlock(BaseModel):
+    """Evento emitido cuando se desbloquea un logro"""
+    player_name: str
+    achievement: AchievementResponse
+    unlocked_at: datetime
+
+
+class PlayerStats(BaseModel):
+    """Estadisticas de jugador para calculo de logros"""
+    player_name: str
+    current_streak: int = 0
+    total_correct: int = 0
+    total_steals: int = 0
+    successful_steals: int = 0
+    category_correct: dict[int, int] = {}  # category_id -> count
+    fastest_answer_seconds: Optional[float] = None
+    questions_500_correct: int = 0