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/services/__init__.py

@@ -2,5 +2,14 @@ from app.services.ai_validator import AIValidator
 from app.services.ai_generator import AIGenerator
 from app.services.game_manager import GameManager
 from app.services.room_manager import RoomManager
+from app.services.replay_manager import ReplayManager
+from app.services.achievement_manager import AchievementManager
 
-__all__ = ["AIValidator", "AIGenerator", "GameManager", "RoomManager"]
+__all__ = [
+    "AIValidator",
+    "AIGenerator",
+    "GameManager",
+    "RoomManager",
+    "ReplayManager",
+    "AchievementManager",
+]