refactor: reorganize project into monorepo with frontend scaffolding

Convert project from single backend to monorepo structure with separate
frontend (Vue 3 + TypeScript + Vite) and backend directories. Updates
all configuration files and build system to support both workspaces.

Ref: T007 (specs/001-modbus-relay-control)

backend/src/infrastructure/mod.rs

@@ -0,0 +1,78 @@
+//! Infrastructure layer - External integrations and adapters
+//!
+//! This module implements the technical infrastructure required by the application,
+//! including external system integrations, persistence, and communication protocols.
+//! All infrastructure depends on domain/application layers through trait implementations.
+//!
+//! # Architecture Principles
+//!
+//! - **Implements domain traits**: Provides concrete implementations of domain interfaces
+//! - **Depends inward**: Depends on domain/application, never the reverse
+//! - **Substitutable**: Different implementations can be swapped without changing domain
+//! - **Framework-specific**: Contains framework and library dependencies (Modbus, SQLx, etc.)
+//!
+//! # Planned Submodules
+//!
+//! ## `modbus` - Modbus RTU over TCP Integration
+//!
+//! - `client`: ModbusRelayController implementation using tokio-modbus
+//! - `mock`: MockRelayController for testing without hardware
+//! - `config`: Modbus connection configuration
+//! - `connection`: Connection pool and health management
+//!
+//! Implements: [`domain::relay::controller::RelayController`](crate::domain)
+//!
+//! ## `persistence` - SQLite Database with SQLx
+//!
+//! - `sqlite_repository`: SqliteRelayLabelRepository implementation
+//! - `schema.sql`: Database schema with relay_labels table
+//! - `migrations`: Database migration scripts (if using sqlx-cli)
+//!
+//! Implements: [`domain::relay::repository::RelayLabelRepository`](crate::domain)
+//!
+//! # Technology Stack
+//!
+//! - **Modbus**: `tokio-modbus` 0.17.0 for async Modbus RTU over TCP
+//! - **Persistence**: `sqlx` 0.8 for compile-time verified SQLite queries
+//! - **Async Runtime**: `tokio` 1.48 (shared with main application)
+//! - **Testing**: `mockall` 0.13 for mock implementations
+//!
+//! # Implementation Pattern
+//!
+//! Each infrastructure adapter:
+//! 1. Implements domain-defined trait (e.g., `RelayController`)
+//! 2. Translates domain types to/from external formats
+//! 3. Handles connection management and retries
+//! 4. Provides error translation (external errors → domain errors)
+//!
+//! # Example: Modbus Controller
+//!
+//! ```rust,ignore
+//! pub struct ModbusRelayController {
+//!     client: Arc<Mutex<ModbusClient>>,
+//!     config: ModbusConfig,
+//! }
+//!
+//! #[async_trait]
+//! impl RelayController for ModbusRelayController {
+//!     async fn read_relay_state(&self, id: RelayId) -> Result<RelayState, ControllerError> {
+//!         let address = self.config.base_address + id.value() as u16;
+//!         let mut client = self.client.lock().await;
+//!
+//!         // Read coil from Modbus device
+//!         let result = client.read_coils(address, 1).await
+//!             .map_err(|e| ControllerError::CommunicationError(e.to_string()))?;
+//!
+//!         // Translate to domain type
+//!         Ok(if result[0] { RelayState::On } else { RelayState::Off })
+//!     }
+//! }
+//! ```
+//!
+//! # References
+//!
+//! - Architecture: `specs/constitution.md` - Dependency Inversion Principle
+//! - Implementation: `specs/001-modbus-relay-control/plan.md` - Infrastructure tasks
+//! - Modbus docs: `docs/Modbus_POE_ETH_Relay.md` - Hardware protocol specification
+
+pub mod persistence;