refactor: 标准化项目结构 frontend/ + backend/ + SQLite + run.sh

- 目录重构: app/ -> frontend/，与 backend/ 并列
- 数据库: 默认 SQLite（零配置），修复 DSN() bug
- run.sh: 一键启动前后端，支持 start/stop/restart/status
- Makefile: install/dev/build/lint/clean 快捷命令
- vite.config.ts: 配置 dev server 端口 5173 + API 代理到 8080
- config.yaml: SQLite 默认配置，开箱即用
- docs/: 完整开发文档（项目结构/API/数据库/规范/部署）
- 修复: GORM foreign key 关联声明
- 修复: go get gorm.io/driver/sqlite 依赖

backend/go.mod

@@ -10,7 +10,8 @@ require (
 	golang.org/x/crypto v0.18.0
 	gorm.io/driver/mysql v1.5.2
 	gorm.io/driver/postgres v1.5.4
-	gorm.io/gorm v1.25.5
+	gorm.io/driver/sqlite v1.6.0
+	gorm.io/gorm v1.30.0
 )
 
 require (
@@ -35,6 +36,7 @@ require (
 	github.com/leodido/go-urn v1.2.4 // indirect
 	github.com/magiconair/properties v1.8.7 // indirect
 	github.com/mattn/go-isatty v0.0.19 // indirect
+	github.com/mattn/go-sqlite3 v1.14.22 // indirect
 	github.com/mitchellh/mapstructure v1.5.0 // indirect
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/modern-go/reflect2 v1.0.2 // indirect
@@ -53,7 +55,7 @@ require (
 	golang.org/x/exp v0.0.0-20230905200255-921286631fa9 // indirect
 	golang.org/x/net v0.19.0 // indirect
 	golang.org/x/sys v0.16.0 // indirect
-	golang.org/x/text v0.14.0 // indirect
+	golang.org/x/text v0.20.0 // indirect
 	google.golang.org/protobuf v1.31.0 // indirect
 	gopkg.in/ini.v1 v1.67.0 // indirect
 	gopkg.in/yaml.v3 v3.0.1 // indirect