feat: 商机跟单新增跟进记录子表（后端+API模块）

- 新增 mini_business_logs 表（business_id/log_date/method/content/created_by）
- MiniBusiness 模型添加 logs relationship（cascade delete）
- 5个端点: GET列表/POST新增/PUT编辑/DELETE删除
- 跟进方式: 电话/微信/上门/邮件/其他
- lifespan 自动建表+索引

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

backend/app/models/__init__.py

@@ -10,6 +10,7 @@ from app.models.daily_note import DailyNote
 from app.models.ai_summary import AISummary
 from app.models.system_config import SystemConfig
 from app.models.leave import Leave
+from app.models.mini_business_log import MiniBusinessLog
 
 __all__ = [
     "User",
@@ -19,6 +20,7 @@ __all__ = [
     "Visit",
     "WorkPlan",
     "MiniBusiness",
+    "MiniBusinessLog",
     "KeyVisit",
     "DailyNote",
     "AISummary",