Refactor MQTT course and bound backend message processing

Move lesson source into the teaching structure, align the course path with the learner's Go and DevOps experience, and add the interactive lesson guides. Add advanced DevOps lifecycle study as a separate section.

Bound MQTT message work and drain accepted jobs during shutdown; align the backend implementation with the revised course through Section 6.

internal/config/config_test.go

@@ -0,0 +1,17 @@
+package config
+
+import "testing"
+
+func TestValidateRejectsDeviceIDThatCannotFormTopic(t *testing.T) {
+	cfg := Config{}
+	cfg.MQTT.URL = "mqtt://127.0.0.1:18883"
+	cfg.MQTT.ClientID = "lab-client"
+	cfg.MQTT.Username = "backend"
+	cfg.MQTT.Password = "lab-password"
+	cfg.MQTT.DeviceID = "device!001"
+	cfg.App.ConnectTimeout = "10s"
+
+	if err := cfg.Validate(); err == nil {
+		t.Fatal("Validate accepted a device ID rejected by the topic contract")
+	}
+}