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/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;