feat(infrastructure): implement SQLite repository for relay labels

Add complete SQLite implementation of RelayLabelRepository trait with
all CRUD operations (get_label, save_label, delete_label, get_all_labels).

Key changes:
- Create infrastructure entities module with RelayLabelRecord struct
- Implement TryFrom traits for converting between database records and domain types
- Add From<sqlx::Error> and From<RelayLabelError> for RepositoryError
- Write comprehensive functional tests covering all repository operations
- Verify proper handling of edge cases (empty results, overwrites, max length)

TDD phase: GREEN - All repository trait tests now passing with SQLite implementation

Ref: T036 (specs/001-modbus-relay-control/tasks.md)

backend/src/domain/relay/repository/mod.rs

@@ -1,7 +1,7 @@
 mod label;
 pub use label::RelayLabelRepository;
 
-use super::types::RelayId;
+use super::types::{RelayId, RelayLabelError};
 
 /// Errors that can occur during repository operations.
 ///
@@ -16,3 +16,15 @@ pub enum RepositoryError {
     #[error("Relay not found: {0}")]
     NotFound(RelayId),
 }
+
+impl From<sqlx::Error> for RepositoryError {
+    fn from(value: sqlx::Error) -> Self {
+        Self::DatabaseError(value.to_string())
+    }
+}
+
+impl From<RelayLabelError> for RepositoryError {
+    fn from(value: RelayLabelError) -> Self {
+        Self::DatabaseError(value.to_string())
+    }
+}

backend/src/domain/relay/types/mod.rs

@@ -3,5 +3,5 @@ mod relaylabel;
 mod relaystate;
 
 pub use relayid::RelayId;
-pub use relaylabel::RelayLabel;
+pub use relaylabel::{RelayLabel, RelayLabelError};
 pub use relaystate::RelayState;

backend/src/domain/relay/types/relaylabel.rs

@@ -8,10 +8,19 @@ use thiserror::Error;
 #[repr(transparent)]
 pub struct RelayLabel(String);
 
+/// Errors that can occur when creating or validating relay labels.
 #[derive(Debug, Error)]
 pub enum RelayLabelError {
+    /// The label string is empty.
+    ///
+    /// Relay labels must contain at least one character.
     #[error("Label cannot be empty")]
     Empty,
+
+    /// The label string exceeds the maximum allowed length.
+    ///
+    /// Contains the actual length of the invalid label.
+    /// Maximum allowed length is 50 characters.
     #[error("Label exceeds maximum length of 50 characters: {0}")]
     TooLong(usize),
 }

backend/src/infrastructure/persistence/entities/mod.rs

@@ -0,0 +1,33 @@
+//! Infrastructure entities for database persistence.
+//!
+//! This module defines entities that directly map to database tables,
+//! providing a clear separation between the persistence layer and the
+//! domain layer. These entities represent raw database records without
+//! domain validation or business logic.
+//!
+//! # Conversion Pattern
+//!
+//! Infrastructure entities implement `TryFrom` traits to convert between
+//! database records and domain types:
+//!
+//! ```rust
+//! # use sta::domain::relay::types::{RelayId, RelayLabel};
+//! # use sta::infrastructure::persistence::entities::relay_label_record::RelayLabelRecord;
+//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
+//! // Database Record -> Domain Types
+//! // ... from database
+//! let record: RelayLabelRecord = RelayLabelRecord { relay_id: 2, label: "label".to_string() };
+//! let (relay_id, relay_label): (RelayId, RelayLabel) = record.try_into()?;
+//!
+//! // Domain Types -> Database Record
+//! let domain_record= RelayLabelRecord::new(relay_id, &relay_label);
+//! # Ok(())
+//! # }
+//! ```
+
+/// Database entity for relay labels.
+///
+/// This module contains the `RelayLabelRecord` struct which represents
+/// a single row in the `RelayLabels` database table, along with conversion
+/// traits to and from domain types.
+pub mod relay_label_record;

backend/src/infrastructure/persistence/entities/relay_label_record.rs

@@ -0,0 +1,62 @@
+use crate::domain::relay::{
+    controller::ControllerError,
+    repository::RepositoryError,
+    types::{RelayId, RelayLabel, RelayLabelError},
+};
+
+/// Database record representing a relay label.
+///
+/// This struct directly maps to the `RelayLabels` table in the
+/// database. It represents the raw data as stored in the database,
+/// without domain validation or business logic.
+#[derive(Debug, Clone, sqlx::FromRow)]
+pub struct RelayLabelRecord {
+    /// The relay ID (1-8) as stored in the database
+    pub relay_id: i64,
+    /// The label text as stored in the database
+    pub label: String,
+}
+
+impl RelayLabelRecord {
+    /// Creates a new `RecordLabelRecord` from domain types.
+    #[must_use]
+    pub fn new(relay_id: RelayId, label: &RelayLabel) -> Self {
+        Self {
+            relay_id: i64::from(relay_id.as_u8()),
+            label: label.as_str().to_string(),
+        }
+    }
+}
+
+impl TryFrom<RelayLabelRecord> for RelayId {
+    type Error = ControllerError;
+
+    fn try_from(value: RelayLabelRecord) -> Result<Self, Self::Error> {
+        let value = u8::try_from(value.relay_id).map_err(|e| {
+            Self::Error::InvalidInput(format!("Got value {} from database: {e}", value.relay_id))
+        })?;
+        Self::new(value)
+    }
+}
+
+impl TryFrom<RelayLabelRecord> for RelayLabel {
+    type Error = RelayLabelError;
+
+    fn try_from(value: RelayLabelRecord) -> Result<Self, Self::Error> {
+        Self::new(value.label)
+    }
+}
+
+impl TryFrom<RelayLabelRecord> for (RelayId, RelayLabel) {
+    type Error = RepositoryError;
+
+    fn try_from(value: RelayLabelRecord) -> Result<Self, Self::Error> {
+        let record_id: RelayId = value
+            .clone()
+            .try_into()
+            .map_err(|e: ControllerError| RepositoryError::DatabaseError(e.to_string()))?;
+        let label: RelayLabel = RelayLabel::new(value.label)
+            .map_err(|e| RepositoryError::DatabaseError(e.to_string()))?;
+        Ok((record_id, label))
+    }
+}

backend/src/infrastructure/persistence/mod.rs

@@ -12,3 +12,5 @@ pub mod label_repository_tests;
 
 /// `SQLite` repository implementation for relay labels.
 pub mod sqlite_repository;
+
+pub mod entities;

backend/src/infrastructure/persistence/sqlite_repository.rs

@@ -1,6 +1,13 @@
-use sqlx::SqlitePool;
+use async_trait::async_trait;
+use sqlx::{SqlitePool, query_as};
 
-use crate::domain::relay::repository::RepositoryError;
+use crate::{
+    domain::relay::{
+        repository::{RelayLabelRepository, RepositoryError},
+        types::{RelayId, RelayLabel},
+    },
+    infrastructure::persistence::entities::relay_label_record::RelayLabelRecord,
+};
 
 /// `SQLite` implementation of the relay label repository.
 ///
@@ -62,3 +69,56 @@ impl SqliteRelayLabelRepository {
         Ok(())
     }
 }
+
+#[async_trait]
+impl RelayLabelRepository for SqliteRelayLabelRepository {
+    async fn get_label(&self, id: RelayId) -> Result<Option<RelayLabel>, RepositoryError> {
+        let id = i64::from(id.as_u8());
+        let result = sqlx::query_as!(
+            RelayLabelRecord,
+            "SELECT * FROM RelayLabels WHERE relay_id = ?1",
+            id
+        )
+        .fetch_optional(&self.pool)
+        .await
+        .map_err(|e| RepositoryError::DatabaseError(e.to_string()))?;
+
+        match result {
+            Some(record) => Ok(Some(record.try_into()?)),
+            None => Ok(None),
+        }
+    }
+
+    async fn save_label(&self, id: RelayId, label: RelayLabel) -> Result<(), RepositoryError> {
+        let record = RelayLabelRecord::new(id, &label);
+        sqlx::query!(
+            "INSERT OR REPLACE INTO RelayLabels (relay_id, label) VALUES (?1, ?2)",
+            record.relay_id,
+            record.label
+        )
+        .execute(&self.pool)
+        .await
+        .map_err(RepositoryError::from)?;
+        Ok(())
+    }
+
+    async fn delete_label(&self, id: RelayId) -> Result<(), RepositoryError> {
+        let id = i64::from(id.as_u8());
+        sqlx::query!("DELETE FROM RelayLabels WHERE relay_id = ?1", id)
+            .execute(&self.pool)
+            .await
+            .map_err(RepositoryError::from)?;
+        Ok(())
+    }
+
+    async fn get_all_labels(&self) -> Result<Vec<(RelayId, RelayLabel)>, RepositoryError> {
+        let result: Vec<RelayLabelRecord> = query_as!(
+            RelayLabelRecord,
+            "SELECT * FROM RelayLabels ORDER BY relay_id"
+        )
+        .fetch_all(&self.pool)
+        .await
+        .map_err(RepositoryError::from)?;
+        result.iter().map(|r| r.clone().try_into()).collect()
+    }
+}