feat(config): 统一加载 .env 并强化本地启动脚本

backend/cmd/migrate-schema/main.go

@@ -8,10 +8,12 @@ import (
 	"strings"
 
 	"infinite-canvas/backend/internal/canvas"
+	"infinite-canvas/backend/internal/config"
 	"infinite-canvas/backend/internal/database"
 )
 
 func main() {
+	config.LoadDotEnv()
 	command := "up"
 	if len(os.Args) > 1 {
 		command = strings.ToLower(strings.TrimSpace(os.Args[1]))