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/util/like.go

@@ -0,0 +1,16 @@
+// Copyright 2026 Arctel.net
+// SPDX-License-Identifier: Apache-2.0
+
+package util
+
+import "strings"
+
+var likeEscaper = strings.NewReplacer(`\`, `\\`, `%`, `\%`, `_`, `\_`)
+
+// EscapeLike escapes SQL LIKE metacharacters (\, %, _) so a user-supplied
+// value matches literally in LIKE patterns. Pair it with an explicit
+// `ESCAPE '\'` clause where the dialect has no backslash default (SQLite);
+// PostgreSQL and ClickHouse treat backslash as the default LIKE escape.
+func EscapeLike(value string) string {
+	return likeEscaper.Replace(value)
+}