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/migrations-clickhouse/00001_initial.sql

@@ -0,0 +1,21 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS w_user_access_logs
+(
+    id          UInt64,
+    user_id     UInt64,
+    path        String,
+    method      String,
+    ip          String,
+    user_agent  String,
+    headers     String,
+    status      Int32,
+    latency     Int64,
+    created_at  DateTime
+)
+ENGINE = MergeTree()
+PARTITION BY toYYYYMM(created_at)
+ORDER BY (created_at, ip, user_id)
+SETTINGS index_granularity = 8192;
+
+-- +goose Down
+DROP TABLE IF EXISTS w_user_access_logs;