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/shutdown.go

@@ -0,0 +1,23 @@
+package dispatch
+
+import (
+	"context"
+	"errors"
+)
+
+func Shutdown(ctx context.Context, pools ...*Pool) error {
+	// Close admission on every pool before waiting for any worker to drain.
+	for _, pool := range pools {
+		if pool != nil {
+			pool.StopAdmission()
+		}
+	}
+	var result error
+	// Share one deadline across the drain so total shutdown time stays bounded.
+	for _, pool := range pools {
+		if pool != nil {
+			result = errors.Join(result, pool.Shutdown(ctx))
+		}
+	}
+	return result
+}