feat(platform): 首页门面三统计 + 运营后台展示模式配置

- 新表 platform_stat_display:每个指标可选 real/manual/random 三种
  展示模式,含建表 + anchor_minutes 两个 alembic migration
- 公开接口 GET /api/v1/platform/stats(无鉴权门面数字,登录前可读)
- 运营后台 GET/PATCH /admin/api/dashboard-display 配置展示模式
- 配套 model/repository/schema,注册 router(app/main+admin/main),
  导出 model,补 docs(api/database 索引及 3 篇详情)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/models/__init__.py

@@ -7,6 +7,7 @@ from app.models.app_config import AppConfig  # noqa: F401
 from app.models.comparison import ComparisonRecord  # noqa: F401
 from app.models.comparison_milestone import ComparisonMilestoneClaim  # noqa: F401
 from app.models.feedback import Feedback  # noqa: F401
+from app.models.platform_stat import PlatformStatDisplay  # noqa: F401
 from app.models.price_observation import PriceObservation  # noqa: F401
 from app.models.price_report import PriceReport  # noqa: F401
 from app.models.savings import SavingsRecord  # noqa: F401