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/mini_business.py

@@ -1,9 +1,13 @@
 import uuid
+from typing import TYPE_CHECKING
 from sqlalchemy import String, Text, ForeignKey
-from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 from app.database import Base
 
+if TYPE_CHECKING:
+    from app.models.mini_business_log import MiniBusinessLog
+
 
 class MiniBusiness(Base):
     __tablename__ = "mini_business"
@@ -17,3 +21,5 @@ class MiniBusiness(Base):
     manager_id: Mapped[uuid.UUID] = mapped_column(UUID(as_uuid=True), ForeignKey("users.id"), index=True)
     expected_revenue_date: Mapped[str] = mapped_column(String(50), default="")
     edit_log: Mapped[list] = mapped_column(JSONB, default=list)
+
+    logs: Mapped[list["MiniBusinessLog"]] = relationship(back_populates="business", cascade="all, delete-orphan")