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/gin_context.go

@@ -0,0 +1,22 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import "github.com/gin-gonic/gin"
+
+// GetFromContext retrieves a typed value from Gin context.
+func GetFromContext[T any](c *gin.Context, key string) (T, bool) {
+	value, exists := c.Get(key)
+	if !exists {
+		var zero T
+		return zero, false
+	}
+	typed, ok := value.(T)
+	return typed, ok
+}
+
+// SetToContext sets a typed value into Gin context.
+func SetToContext[T any](c *gin.Context, key string, value T) {
+	c.Set(key, value)
+}