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/schemas/mini_business_log.py

@@ -0,0 +1,33 @@
+"""MiniBusinessLog Pydantic schemas."""
+
+from datetime import date, datetime
+from uuid import UUID
+from typing import Optional
+from pydantic import BaseModel, Field
+
+FOLLOW_UP_METHODS = ["电话", "微信", "上门", "邮件", "其他"]
+
+
+class MiniBusinessLogCreate(BaseModel):
+    log_date: date
+    method: str = Field(default="电话", pattern="^(电话|微信|上门|邮件|其他)$")
+    content: str = ""
+
+
+class MiniBusinessLogUpdate(BaseModel):
+    log_date: Optional[date] = None
+    method: Optional[str] = Field(default=None, pattern="^(电话|微信|上门|邮件|其他)$")
+    content: Optional[str] = None
+
+
+class MiniBusinessLogOut(BaseModel):
+    id: UUID
+    business_id: UUID
+    log_date: date
+    method: str
+    content: str
+    created_by: UUID
+    created_by_name: str = ""
+    created_at: datetime | None = None
+
+    model_config = {"from_attributes": True}