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_log.py

@@ -0,0 +1,30 @@
+"""MiniBusinessLog — follow-up log entries for mini business opportunities."""
+
+import uuid
+from datetime import date, datetime
+from sqlalchemy import String, Date, DateTime, ForeignKey, Text, func
+from sqlalchemy.dialects.postgresql import UUID
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from app.database import Base
+
+
+class MiniBusinessLog(Base):
+    __tablename__ = "mini_business_logs"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    business_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("mini_business.id", ondelete="CASCADE"), index=True
+    )
+    log_date: Mapped[date] = mapped_column(Date)
+    method: Mapped[str] = mapped_column(String(20), default="电话")
+    content: Mapped[str] = mapped_column(Text, default="")
+    created_by: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id")
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now()
+    )
+
+    business: Mapped["MiniBusiness"] = relationship(back_populates="logs")