feat: 添加七牛云 SDK 依赖与 QiniuConfig 配置

- go.mod 新增 github.com/qiniu/go-sdk/v7 v7.26.12
- config.go 新增 QiniuConfig 结构体，绑定 GEN2D_QINIU_* 环境变量
- .env.example 补充七牛云环境变量模板

backend/go.mod

@@ -7,6 +7,7 @@ require (
 	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/qiniu/go-sdk/v7 v7.26.12
 	github.com/spf13/viper v1.21.0
 	golang.org/x/crypto v0.48.0
 	gorm.io/driver/sqlite v1.6.0
@@ -14,6 +15,8 @@ require (
 )
 
 require (
+	github.com/BurntSushi/toml v1.3.2 // indirect
+	github.com/alex-ant/gomath v0.0.0-20160516115720-89013a210a82 // indirect
 	github.com/bahlo/generic-list-go v0.2.0 // indirect
 	github.com/buger/jsonparser v1.1.1 // indirect
 	github.com/bytedance/gopkg v0.1.3 // indirect
@@ -24,6 +27,7 @@ require (
 	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/gammazero/toposort v0.1.1 // 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
@@ -31,6 +35,7 @@ require (
 	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/gofrs/flock v0.8.1 // 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
@@ -65,8 +70,10 @@ require (
 	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
+	golang.org/x/sync v0.19.0 // 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
 	gopkg.in/yaml.v3 v3.0.1 // indirect
+	modernc.org/fileutil v1.0.0 // indirect
 )