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/internal/config/config.go

@@ -1,6 +1,8 @@
 package config
 
 import (
+	"fmt"
+
 	"github.com/spf13/viper"
 	"go.uber.org/zap"
 )
@@ -60,7 +62,8 @@ func Load() *Config {
 	viper.SetDefault("mode", "development")
 	viper.SetDefault("port", "8080")
 	viper.SetDefault("log_level", "info")
-	viper.SetDefault("database.driver", "mysql")
+	viper.SetDefault("database.driver", "sqlite")
+	viper.SetDefault("database.dbname", "data/aiops.db")
 	viper.SetDefault("database.host", "localhost")
 	viper.SetDefault("database.port", 3306)
 	viper.SetDefault("database.charset", "utf8mb4")
@@ -90,7 +93,11 @@ func Load() *Config {
 // DSN 返回数据库连接字符串
 func (d *DatabaseConfig) DSN() string {
 	if d.Driver == "mysql" {
-		return d.User + ":" + d.Password + "@tcp(" + d.Host + ":" + string(rune(d.Port)) + ")/" + d.DBName + "?charset=" + d.Charset + "&parseTime=True&loc=Local"
+		return fmt.Sprintf("%s:%s@tcp(%s:%d)/%s?charset=%s&parseTime=True&loc=Local",
+			d.User, d.Password, d.Host, d.Port, d.DBName, d.Charset)
+	}
+	if d.Driver == "sqlite" {
+		return d.DBName
 	}
 	return ""
 }