feat(persistence): add SQLite schema for relay labels table

Create infrastructure/persistence/schema.sql with relay_labels table
definition. Table enforces relay_id range (1-8) and label length (max 50).

Ref: T005 (specs/001-modbus-relay-control)

specs/001-modbus-relay-control/tasks.md

@@ -35,7 +35,7 @@
   - **Test**: Settings::new() loads config without errors
   - **Complexity**: Low | **Uncertainty**: Low
 
-- [ ] **T005** [P] [Setup] [TDD] Add SQLite schema file
+- [x] **T005** [P] [Setup] [TDD] Add SQLite schema file
   - Create infrastructure/persistence/schema.sql with relay_labels table
   - Table: `relay_labels (relay_id INTEGER PRIMARY KEY CHECK(relay_id BETWEEN 1 AND 8), label TEXT NOT NULL CHECK(length(label) <= 50))`
   - **Test**: Schema file syntax is valid SQL