test: 后端自动化测试套件

Service 单元测试（SQLite in-memory）：
- auth: 登录成功/失败/禁用账号，token 刷新
- stock: 入库审核、出库审核（含库存不足）、单号生成
- license: 激活、设备绑定验证、过期检测

Handler 集成测试（httptest + SQLite）：
- auth: 登录 API 成功/401 场景
- product: CRUD 完整流程 + hotel_id 隔离验证
- stock_in: 创建→提交→审核完整流程
- stock_out: 完整流程 + 库存不足 400
- inventory: 库存查询/过滤/盘点创建
- warehouse: CRUD

覆盖率：service 90.4%，handler 56.1%
共 50 个测试用例，全部 PASS

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/go.mod

@@ -6,9 +6,11 @@ require (
 	github.com/gin-gonic/gin v1.12.0
 	github.com/golang-jwt/jwt/v5 v5.3.1
 	github.com/spf13/viper v1.21.0
+	github.com/stretchr/testify v1.11.1
 	github.com/xuri/excelize/v2 v2.10.1
 	golang.org/x/crypto v0.49.0
 	gorm.io/driver/mysql v1.6.0
+	gorm.io/driver/sqlite v1.6.0
 	gorm.io/gorm v1.31.1
 )
 
@@ -18,6 +20,7 @@ require (
 	github.com/bytedance/sonic v1.15.0 // indirect
 	github.com/bytedance/sonic/loader v0.5.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.12 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect
@@ -34,9 +37,11 @@ require (
 	github.com/klauspost/cpuid/v2 v2.3.0 // indirect
 	github.com/leodido/go-urn v1.4.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
+	github.com/mattn/go-sqlite3 v1.14.22 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
 	github.com/pelletier/go-toml/v2 v2.2.4 // indirect
+	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/quic-go/qpack v0.6.0 // indirect
 	github.com/quic-go/quic-go v0.59.0 // indirect
 	github.com/richardlehane/mscfb v1.0.6 // indirect
@@ -59,4 +64,5 @@ require (
 	golang.org/x/sys v0.42.0 // indirect
 	golang.org/x/text v0.35.0 // indirect
 	google.golang.org/protobuf v1.36.10 // indirect
+	gopkg.in/yaml.v3 v3.0.1 // indirect
 )