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/risk_control/logstore/logstore.go

@@ -0,0 +1,46 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+// Package logstore abstracts user access-log storage across ClickHouse, PostgreSQL and SQLite.
+package logstore
+
+import (
+	"context"
+	"errors"
+	"time"
+)
+
+// ErrMigrating 表示日志数据库正在迁移，当前禁止写入。
+var ErrMigrating = errors.New("log database is migrating, writes are disabled")
+
+// UserAccessLogStore 用户访问日志（w_user_access_logs）。
+type UserAccessLogStore interface {
+	BatchInsert(ctx context.Context, logs []UserAccessLog) error
+	DeleteAll(ctx context.Context) (int64, error)
+	DeleteBefore(ctx context.Context, cutoff time.Time) (int64, error)
+	Count(ctx context.Context, filter AccessLogFilter) (uint64, error)
+	List(ctx context.Context, filter AccessLogFilter, page, pageSize int) ([]UserAccessLog, uint64, error)
+	GetDailyTrend(ctx context.Context, days int) ([]DailyTrend, error)
+	GetBrowserDistribution(ctx context.Context, startTime time.Time) ([]BrowserShare, error)
+	GetTopActiveUsers(ctx context.Context, startTime time.Time, limit int) ([]TopUser, error)
+	ListForMigration(ctx context.Context, afterID uint64, limit int) ([]UserAccessLog, error)
+	MigrationRange(ctx context.Context) (from, to time.Time, err error)
+	EnsurePartitions(ctx context.Context, from, to time.Time) error
+	// DropEmptyPartitions 幂等清理 PG 空分区表：删除 before 月份之前、且无任何数据的按月分区；
+	// CH/SQLite 为 no-op。
+	DropEmptyPartitions(ctx context.Context, before time.Time) error
+	// DropExpiredPartitions 直接删除完全过期的 PG 整月分区（候选为月份早于 cutoff 月的分区，
+	// 删除前校验分区内无保留期内数据，避免时区偏移下误删；迁移冻结期间拒绝执行）；CH/SQLite 为 no-op。
+	DropExpiredPartitions(ctx context.Context, cutoff time.Time) error
+}
+
+// StatusStore 日志库状态。
+type StatusStore interface {
+	ActiveDatabase(ctx context.Context) (string, error)
+}
+
+// Store 当前生效日志库。
+type Store struct {
+	UserAccessLogs UserAccessLogStore
+	Status         StatusStore
+}