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>

frontend/src/api/miniBusiness.ts

@@ -13,4 +13,17 @@ export const miniBusinessApi = {
   delete(id: string) {
     return api.delete(`/mini-business/${id}`)
   },
+  // Follow-up logs
+  getLogs(businessId: string) {
+    return api.get(`/mini-business/${businessId}/logs`)
+  },
+  createLog(businessId: string, data: any) {
+    return api.post(`/mini-business/${businessId}/logs`, data)
+  },
+  updateLog(logId: string, data: any) {
+    return api.put(`/mini-business/logs/${logId}`, data)
+  },
+  deleteLog(logId: string) {
+    return api.delete(`/mini-business/logs/${logId}`)
+  },
 }