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)

.env.example

@@ -0,0 +1 @@
+DATABASE_URL=sqlite://db.sqlite

.gitignore

@@ -40,3 +40,4 @@ result
 # Added by cargo
 
 /target
+*.sqlite

migrations/0001_relay-labels.down.sql

@@ -0,0 +1 @@
+DROP TABLE IF EXISTS RelayLabels;

migrations/0001_relay-labels.up.sql

@@ -0,0 +1,4 @@
+CREATE TABLE IF NOT EXISTS RelayLabels (
+  relay_id INTEGER PRIMARY KEY CHECK(relay_id BETWEEN 1 AND 8),
+  label TEXT NOT NULL CHECK(length(label) <= 50)
+);

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