feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/go.mod

@@ -3,16 +3,29 @@ module nex/backend
 go 1.26.2
 
 require (
+	github.com/gin-gonic/gin v1.12.0
+	github.com/google/uuid v1.6.0
+	github.com/pressly/goose/v3 v3.27.0
+	github.com/stretchr/testify v1.11.1
+	go.uber.org/zap v1.27.1
+	gopkg.in/lumberjack.v2 v2.0.0
+	gopkg.in/yaml.v3 v3.0.1
+	gorm.io/driver/sqlite v1.6.0
+	gorm.io/gorm v1.31.1
+)
+
+require (
+	github.com/BurntSushi/toml v1.6.0 // indirect
 	github.com/bytedance/gopkg v0.1.3 // indirect
 	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/gabriel-vasile/mimetype v1.4.12 // indirect
+	github.com/davecgh/go-spew v1.1.1 // indirect
+	github.com/gabriel-vasile/mimetype v1.4.13 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect
-	github.com/gin-gonic/gin v1.12.0 // indirect
 	github.com/go-playground/locales v0.14.1 // indirect
 	github.com/go-playground/universal-translator v0.18.1 // indirect
-	github.com/go-playground/validator/v10 v10.30.1 // indirect
+	github.com/go-playground/validator/v10 v10.30.2 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/goccy/go-yaml v1.19.2 // indirect
 	github.com/jinzhu/inflection v1.0.0 // indirect
@@ -22,20 +35,25 @@ require (
 	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/mfridman/interpolate v0.0.2 // 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/sethvargo/go-retry v0.3.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.3.1 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
+	go.uber.org/multierr v1.11.0 // indirect
 	golang.org/x/arch v0.22.0 // indirect
-	golang.org/x/crypto v0.48.0 // indirect
+	golang.org/x/crypto v0.49.0 // indirect
 	golang.org/x/net v0.51.0 // indirect
-	golang.org/x/sys v0.41.0 // indirect
-	golang.org/x/text v0.34.0 // indirect
-	google.golang.org/protobuf v1.36.10 // indirect
-	gorm.io/driver/sqlite v1.6.0 // indirect
-	gorm.io/gorm v1.31.1 // indirect
+	golang.org/x/sync v0.20.0 // indirect
+	golang.org/x/sys v0.42.0 // indirect
+	golang.org/x/text v0.35.0 // indirect
+	google.golang.org/protobuf v1.36.11 // indirect
+	gopkg.in/natefinch/lumberjack.v2 v2.2.1 // indirect
+	gopkg.in/yaml.v2 v2.4.0 // indirect
 )