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/config/config_test.go

@@ -0,0 +1,17 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package config
+
+import "testing"
+
+func TestApplyEnvOverridesRedisMaintNotifications(t *testing.T) {
+	t.Setenv("REDIS_MAINT_NOTIFICATIONS", "true")
+
+	cfg := &configModel{}
+	applyEnvOverrides(cfg)
+
+	if !cfg.Redis.MaintNotifications {
+		t.Fatal("REDIS_MAINT_NOTIFICATIONS=true was not applied")
+	}
+}