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/dispatch/receiver.go

@@ -0,0 +1,38 @@
+package dispatch
+
+import (
+	"context"
+	"fmt"
+
+	"example.com/mqtt5-learning/internal/contracts"
+	"example.com/mqtt5-learning/internal/mqtt"
+)
+
+type Processor func(context.Context, mqtt.Message) error
+
+// Receiver validates the route, selects a pool, and copies payload ownership.
+type Receiver struct {
+	Telemetry *Pool
+	Important *Pool
+	Process   Processor
+}
+
+func (r *Receiver) Submit(ctx context.Context, message mqtt.Message) (Outcome, error) {
+	if r.Process == nil {
+		return Stopping, fmt.Errorf("message processor is required")
+	}
+	topic, err := contracts.ParseTopic(message.Topic)
+	if err != nil {
+		return Stopping, err
+	}
+	pool := r.Telemetry
+	if topic.Kind == contracts.KindCommands || topic.Kind == contracts.KindResponses || topic.Kind == contracts.KindSales {
+		pool = r.Important
+	}
+	if pool == nil {
+		return Stopping, fmt.Errorf("no queue configured for topic kind %q", topic.Kind)
+	}
+	copyOfMessage := message
+	copyOfMessage.Payload = append([]byte(nil), message.Payload...)
+	return pool.Submit(ctx, func(jobCtx context.Context) error { return r.Process(jobCtx, copyOfMessage) })
+}