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/label.rs

@@ -0,0 +1,41 @@
+use async_trait::async_trait;
+
+use crate::domain::relay::types::{RelayId, RelayLabel};
+
+use super::RepositoryError;
+
+/// Repository trait for persisting and retrieving relay labels.
+///
+/// This trait abstracts data persistence operations for relay labels,
+/// enabling different storage implementations (e.g., `SQLite`, `PostgreSQL`, in-memory).
+/// Implementations must be thread-safe (`Send + Sync`) for use in async contexts.
+#[async_trait]
+pub trait RelayLabelRepository: Send + Sync {
+    /// Retrieves the label for a specific relay.
+    ///
+    /// Returns `None` if no label has been set for the relay.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn get_label(&self, id: RelayId) -> Result<Option<RelayLabel>, RepositoryError>;
+
+    /// Saves or updates the label for a specific relay.
+    ///
+    /// If a label already exists for the relay, it will be overwritten.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn save_label(&self, id: RelayId, label: RelayLabel) -> Result<(), RepositoryError>;
+
+    /// Retrieves all relay labels from the repository.
+    ///
+    /// Returns a vector of tuples containing relay IDs and their corresponding labels.
+    /// Relays without labels are not included in the result.
+    ///
+    /// # Errors
+    ///
+    /// Returns `RepositoryError::DatabaseError` if the database operation fails.
+    async fn get_all_labels(&self) -> Result<Vec<(RelayId, RelayLabel)>, RepositoryError>;
+}