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/domain/relay/repository/mod.rs

@@ -0,0 +1,18 @@
+mod label;
+pub use label::RelayLabelRepository;
+
+use super::types::RelayId;
+
+/// Errors that can occur during repository operations.
+///
+/// This enum provides structured error handling for all data persistence
+/// operations related to relay management.
+#[derive(Debug, thiserror::Error)]
+pub enum RepositoryError {
+    /// A database operation failed with the given error message.
+    #[error("Database error: {0}")]
+    DatabaseError(String),
+    /// The requested relay was not found in the repository.
+    #[error("Relay not found: {0}")]
+    NotFound(RelayId),
+}