test(infrastructure): implement MockRelayLabelRepository for testing

Create in-memory mock implementation of RelayLabelRepository trait
using HashMap with Arc<Mutex<>> for thread-safe concurrent access.
Includes 8 comprehensive tests covering all trait methods and edge
cases.

Also refactor domain repository module structure to support multiple
repository types (repository.rs → repository/label.rs + mod.rs).

TDD phase: Combined red-green (tests + implementation)

Ref: T037, T038

backend/src/infrastructure/persistence/mod.rs

@@ -3,5 +3,8 @@
 //! This module contains the concrete implementations of repository traits
 //! for data persistence, including SQLite-based storage for relay labels.
 
+/// Mock repository implementation for testing.
+pub mod label_repository;
+
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;