feat(achievement): 添加指标注册表与 16 个内置指标

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

achievement/apps.py

@@ -4,3 +4,7 @@ from django.apps import AppConfig
 class AchievementConfig(AppConfig):
     default_auto_field = "django.db.models.BigAutoField"
     name = "achievement"
+
+    def ready(self):
+        # 导入以触发 @metric 装饰器把指标注册进 METRIC_REGISTRY
+        import achievement.metrics  # noqa: F401