feat(config): 重构配置层为 Viper + YAML + .env 三级加载

- config 改用 Viper 读取 YAML + 绑定环境变量 + godotenv 加载 .env
- 新增 LLM / ImageGen / JWT / Database / Server 分组配置结构
- 新增 backend/.env.example 环境变量模板
- .gitignore 添加 backend/.env 和 backend/main

backend/go.mod

@@ -5,6 +5,9 @@ go 1.26.3
 require (
 	github.com/cloudwego/eino v0.8.13
 	github.com/gin-gonic/gin v1.12.0
+	github.com/golang-jwt/jwt/v5 v5.3.1
+	github.com/joho/godotenv v1.5.1
+	github.com/spf13/viper v1.21.0
 	golang.org/x/crypto v0.48.0
 	gorm.io/driver/sqlite v1.6.0
 	gorm.io/gorm v1.31.1
@@ -19,14 +22,15 @@ require (
 	github.com/cloudwego/base64x v0.1.6 // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
 	github.com/eino-contrib/jsonschema v1.0.3 // 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
 	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-viper/mapstructure/v2 v2.4.0 // indirect
 	github.com/goccy/go-json v0.10.5 // indirect
 	github.com/goccy/go-yaml v1.19.2 // indirect
-	github.com/golang-jwt/jwt/v5 v5.3.1 // indirect
 	github.com/google/uuid v1.6.0 // indirect
 	github.com/goph/emperror v0.17.2 // indirect
 	github.com/jinzhu/inflection v1.0.0 // indirect
@@ -44,13 +48,20 @@ require (
 	github.com/pkg/errors v0.9.1 // indirect
 	github.com/quic-go/qpack v0.6.0 // indirect
 	github.com/quic-go/quic-go v0.59.0 // indirect
+	github.com/sagikazarmark/locafero v0.11.0 // indirect
 	github.com/sirupsen/logrus v1.9.3 // indirect
 	github.com/slongfield/pyfmt v0.0.0-20220222012616-ea85ff4c361f // indirect
+	github.com/sourcegraph/conc v0.3.1-0.20240121214520-5f936abd7ae8 // indirect
+	github.com/spf13/afero v1.15.0 // indirect
+	github.com/spf13/cast v1.10.0 // indirect
+	github.com/spf13/pflag v1.0.10 // indirect
+	github.com/subosito/gotenv v1.6.0 // indirect
 	github.com/twitchyliquid64/golang-asm v0.15.1 // indirect
 	github.com/ugorji/go/codec v1.3.1 // indirect
 	github.com/wk8/go-ordered-map/v2 v2.1.8 // indirect
 	github.com/yargevad/filepathx v1.0.0 // indirect
 	go.mongodb.org/mongo-driver/v2 v2.5.0 // indirect
+	go.yaml.in/yaml/v3 v3.0.4 // indirect
 	golang.org/x/arch v0.22.0 // indirect
 	golang.org/x/exp v0.0.0-20230713183714-613f0c0eb8a1 // indirect
 	golang.org/x/net v0.51.0 // indirect