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/plugins/domain/cap/middleware.go

@@ -0,0 +1,38 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package cap
+
+import (
+	"github.com/gin-gonic/gin"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/response"
+)
+
+// VerifyMiddleware returns a Gin middleware that checks and consumes the X-Cap-Token header.
+func VerifyMiddleware(mgr *Manager, scope string) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if !ProtectionEnabled(c.Request.Context()) {
+			c.Next()
+			return
+		}
+		if mgr == nil {
+			response.AbortUnauthorized(c, errCapTokenInvalidOrExpired)
+			return
+		}
+
+		token := c.GetHeader("X-Cap-Token")
+		if token == "" {
+			response.AbortUnauthorized(c, errCapTokenMissing)
+			return
+		}
+
+		valid, err := mgr.VerifyToken(c.Request.Context(), token, scope)
+		if err != nil || !valid {
+			response.AbortUnauthorized(c, errCapTokenInvalidOrExpired)
+			return
+		}
+
+		c.Next()
+	}
+}