feat: 企微应用租户下拉选择 + corp_id 自动填入

- 租户表增加 corp_id 字段（企业微信企业ID）
- 租户管理页面支持配置 corp_id
- 企微应用页面租户从下拉选择
- 选择租户后自动填入 corp_id 并禁用编辑
- 提示用户如租户未配置 corp_id 需先去配置

backend/app/routers/tenants.py

@@ -20,6 +20,7 @@ router = APIRouter(prefix="/tenants", tags=["租户管理"])
 class TenantCreate(BaseModel):
     code: str
     name: str
+    corp_id: Optional[str] = None  # 企业微信企业ID
     contact_info: Optional[dict] = None
     status: str = "active"
     expired_at: Optional[date] = None
@@ -27,6 +28,7 @@ class TenantCreate(BaseModel):
 
 class TenantUpdate(BaseModel):
     name: Optional[str] = None
+    corp_id: Optional[str] = None  # 企业微信企业ID
     contact_info: Optional[dict] = None
     status: Optional[str] = None
     expired_at: Optional[date] = None
@@ -81,6 +83,7 @@ async def list_tenants(
                 "id": t.id,
                 "code": t.code,
                 "name": t.name,
+                "corp_id": t.corp_id,
                 "contact_info": t.contact_info,
                 "status": t.status,
                 "expired_at": t.expired_at,
@@ -120,6 +123,7 @@ async def get_tenant(
         "id": tenant.id,
         "code": tenant.code,
         "name": tenant.name,
+        "corp_id": tenant.corp_id,
         "contact_info": tenant.contact_info,
         "status": tenant.status,
         "expired_at": tenant.expired_at,
@@ -159,6 +163,7 @@ async def create_tenant(
     tenant = Tenant(
         code=data.code,
         name=data.name,
+        corp_id=data.corp_id,
         contact_info=data.contact_info,
         status=data.status,
         expired_at=data.expired_at