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/00001_initial.sql

@@ -0,0 +1,20 @@
+-- +goose Up
+CREATE TABLE IF NOT EXISTS w_user_access_logs (
+    id          BIGINT NOT NULL,
+    user_id     BIGINT NOT NULL DEFAULT 0,
+    path        VARCHAR(2048) NOT NULL DEFAULT '',
+    method      VARCHAR(16) NOT NULL DEFAULT '',
+    ip          VARCHAR(128) NOT NULL DEFAULT '',
+    user_agent  TEXT NOT NULL DEFAULT '',
+    headers     TEXT NOT NULL DEFAULT '',
+    status      INTEGER NOT NULL DEFAULT 0,
+    latency     BIGINT NOT NULL DEFAULT 0,
+    created_at  TIMESTAMPTZ NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    PRIMARY KEY (id, created_at)
+) PARTITION BY RANGE (created_at);
+
+CREATE INDEX IF NOT EXISTS idx_w_user_access_logs_user_id ON w_user_access_logs (user_id, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_w_user_access_logs_created_at ON w_user_access_logs (created_at DESC);
+
+-- +goose Down
+DROP TABLE IF EXISTS w_user_access_logs;