refactor(layout): consolidate backend codebase into backend/ package and clean root directory

- Moved cmd/, core/, plugins/, pkg/, downstream/, and main.go into backend/ directory
- Batch updated all Go source files to import github.com/Rain-kl/Wavelet/backend/...
- Updated Makefile, scripts/swagger.sh, architecture guards, and platform skills
- Passed all quality gates (100% tests, 0 lint issues, clean build)

backend/pkg/util/goroutine.go

@@ -0,0 +1,31 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import (
+	"log/slog"
+	"runtime"
+	"runtime/debug"
+)
+
+// Go runs fn in a new goroutine and recovers panics, so a background task
+// cannot crash the whole process. The panic is logged together with the
+// util.Go call site. Use it for every fire-and-forget / long-lived
+// background goroutine; HTTP handlers are already covered by gin.Recovery.
+func Go(fn func()) {
+	pc, file, line, _ := runtime.Caller(1)
+	go func() {
+		defer func() {
+			if r := recover(); r != nil {
+				slog.Error("panic recovered in background goroutine",
+					"caller", runtime.FuncForPC(pc).Name(),
+					"file", file,
+					"line", line,
+					"panic", r,
+					"stack", string(debug.Stack()))
+			}
+		}()
+		fn()
+	}()
+}