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)

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)
+);