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/plugin.go

@@ -0,0 +1,62 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package cap provides the proof-of-work (PoW) CAPTCHA verification domain plugin for Cordis.
+package cap
+
+import (
+	"github.com/Rain-kl/Wavelet/backend/core"
+	"github.com/Rain-kl/Wavelet/backend/core/extpoints"
+)
+
+// Plugin implements core.Plugin to provide CAPTCHA generation, validation, and route protection.
+type Plugin struct{}
+
+// New creates a new cap domain plugin.
+func New() *Plugin {
+	return &Plugin{}
+}
+
+// Name returns the unique identifier for the cap domain plugin.
+func (p *Plugin) Name() string {
+	return "cap"
+}
+
+// Manifest returns the plugin metadata.
+func (p *Plugin) Manifest() core.Manifest {
+	return core.Manifest{
+		Name:        "cap",
+		Version:     "1.0.0",
+		Description: "Proof-of-work CAPTCHA challenge and verification domain plugin",
+		Author:      "Wavelet Team",
+	}
+}
+
+// Apply registers the cap routes and settings into the Context.
+func (p *Plugin) Apply(ctx *core.Context) error {
+	// Register HTTP Routes
+	capGroup := ctx.Router().Group("/api/v1/cap")
+	{
+		capGroup.GET("/challenge", Challenge)
+		capGroup.POST("/challenge", Challenge)
+		capGroup.POST("/redeem", Redeem)
+	}
+
+	// Register Settings Schemas
+	ctx.Settings().Register(extpoints.SettingSchema{
+		Key:         "cap.login_enabled",
+		Default:     false,
+		Description: "Whether to require CAPTCHA verification for user login",
+		Type:        "boolean",
+		Category:    "security",
+	})
+	ctx.Settings().Register(extpoints.SettingSchema{
+		Key:         "cap.challenge_count",
+		Default:     1,
+		Description: "Number of PoW puzzle challenges to solve",
+		Type:        "integer",
+		Category:    "security",
+	})
+
+	return nil
+}