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;