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/__init__.py

@@ -8,10 +8,20 @@ from app.schemas.game import (
     StealAttempt
 )
 from app.schemas.admin import AdminCreate, AdminLogin, Token
+from app.schemas.achievement import (
+    AchievementBase,
+    AchievementCreate,
+    AchievementResponse,
+    PlayerAchievement,
+    AchievementUnlock,
+    PlayerStats
+)
 
 __all__ = [
     "QuestionCreate", "QuestionUpdate", "QuestionResponse",
     "RoomCreate", "RoomJoin", "PlayerInfo", "GameState",
     "AnswerSubmit", "StealAttempt",
-    "AdminCreate", "AdminLogin", "Token"
+    "AdminCreate", "AdminLogin", "Token",
+    "AchievementBase", "AchievementCreate", "AchievementResponse",
+    "PlayerAchievement", "AchievementUnlock", "PlayerStats"
 ]