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/main.py

@@ -5,7 +5,7 @@ from sqlalchemy import select
 from app.config import settings
 from app.database import engine, Base, async_session
 from app.api import router as api_router
-from app.api import auth, users, customers, visits, work_plans, mini_business, key_visits
+from app.api import auth, users, customers, visits, work_plans, mini_business, mini_business_logs, key_visits
 from app.api import dashboard, upload, export, import_data, wecom, daily_notes, ai_summary, system_config, leaves
 from app.models.system_config import SystemConfig
 from app.services.holidays import refresh_holidays
@@ -74,6 +74,24 @@ async def lifespan(app: FastAPI):
         await conn.run_sync(lambda c: c.exec_driver_sql(
             "CREATE INDEX IF NOT EXISTS idx_leaves_dates ON leaves(start_date, end_date)"
         ))
+        # mini_business_logs table for v0.8
+        await conn.run_sync(lambda c: c.exec_driver_sql(
+            "CREATE TABLE IF NOT EXISTS mini_business_logs ("
+            "  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),"
+            "  business_id UUID NOT NULL REFERENCES mini_business(id) ON DELETE CASCADE,"
+            "  log_date DATE NOT NULL,"
+            "  method VARCHAR(20) NOT NULL DEFAULT '电话',"
+            "  content TEXT NOT NULL DEFAULT '',"
+            "  created_by UUID NOT NULL REFERENCES users(id),"
+            "  created_at TIMESTAMPTZ DEFAULT now()"
+            ")"
+        ))
+        await conn.run_sync(lambda c: c.exec_driver_sql(
+            "CREATE INDEX IF NOT EXISTS idx_mbl_business_id ON mini_business_logs(business_id)"
+        ))
+        await conn.run_sync(lambda c: c.exec_driver_sql(
+            "CREATE INDEX IF NOT EXISTS idx_mbl_log_date ON mini_business_logs(log_date)"
+        ))
 
     # Read notification_time from DB (or use default 17:30)
     notification_hour, notification_minute = 17, 30
@@ -123,6 +141,7 @@ app.include_router(customers.router, prefix="/api")
 app.include_router(visits.router, prefix="/api")
 app.include_router(work_plans.router, prefix="/api")
 app.include_router(mini_business.router, prefix="/api")
+app.include_router(mini_business_logs.router, prefix="/api")
 app.include_router(key_visits.router, prefix="/api")
 app.include_router(dashboard.router, prefix="/api")
 app.include_router(leaves.router, prefix="/api")