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/idgen/snowflake.go

@@ -0,0 +1,46 @@
+// Copyright 2025 linux.do
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package idgen 提供分布式 ID 生成器
+package idgen
+
+import (
+	"fmt"
+	"log"
+
+	"github.com/Rain-kl/Wavelet/backend/pkg/config"
+	"github.com/bwmarrin/snowflake"
+)
+
+// 2025-12-01 00:00:00 UTC 的毫秒时间戳
+const epoch int64 = 1764547200000
+
+const maxNegativeIDRetries = 3
+
+var node *snowflake.Node
+
+func init() {
+	snowflake.Epoch = epoch
+
+	nodeID := config.Config.App.NodeID
+	var err error
+	node, err = snowflake.NewNode(nodeID)
+	if err != nil {
+		log.Fatalf("[Snowflake] init failed: %v\n", err)
+	}
+	log.Printf("[Snowflake] initialized with node ID: %d, epoch: 2025-12-01\n", nodeID)
+}
+
+// NextUint64ID 生成下一个分布式唯一 ID。
+// 理论上不应出现负值；若出现则最多重试 maxNegativeIDRetries 次，仍失败则 panic。
+func NextUint64ID() uint64 {
+	for attempt := 1; attempt <= maxNegativeIDRetries; attempt++ {
+		id := node.Generate().Int64()
+		if id >= 0 {
+			return uint64(id)
+		}
+		log.Printf("[Snowflake] generated negative ID: %d (attempt %d/%d)", id, attempt, maxNegativeIDRetries)
+	}
+	panic(fmt.Sprintf("[Snowflake] generated negative ID after %d attempts", maxNegativeIDRetries))
+}

backend/pkg/idgen/snowflake_test.go

@@ -0,0 +1,15 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package idgen
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestNextUint64ID(t *testing.T) {
+	id := NextUint64ID()
+	assert.NotZero(t, id)
+}