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/infra/database/postgres_logger_test.go

@@ -0,0 +1,39 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package database
+
+import (
+	"testing"
+
+	gormLogger "gorm.io/gorm/logger"
+)
+
+func TestParseLogLevel(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name            string
+		configuredLevel string
+		want            gormLogger.LogLevel
+	}{
+		{
+			name:            "debug enables SQL trace processing",
+			configuredLevel: "debug",
+			want:            gormLogger.Info,
+		},
+		{
+			name:            "development preserves configured level",
+			configuredLevel: "warn",
+			want:            gormLogger.Warn,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := parseLogLevel(tt.configuredLevel); got != tt.want {
+				t.Fatalf("parseLogLevel() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}