feat(settings): add modbus and relay configuration structs

Add ModbusSettings with host, port, slave_id, and timeout_secs fields.
Add RelaySettings with label_max_length field.
Integrate both into Settings struct and load from settings/base.yaml
with test Modbus TCP configuration.

Ref: T003 (specs/001-modbus-relay-control)

settings/base.yaml

@@ -6,3 +6,12 @@ rate_limit:
   enabled: true
   burst_size: 10
   per_seconds: 60
+
+modbus:
+  host: "192.168.0.200"
+  port: 502
+  slave_id: 0
+  timeout_secs: 5
+
+relay:
+  label_max_length: 8