Add release workflow, LICENSE, and version stamping

Tag-triggered workflow builds multi-platform binaries, pushes a multi-arch
Docker image to GHCR, bumps and releases the Helm chart, and creates a
GitHub release with all binary artifacts. Adds GPL-3.0 LICENSE and version
variable stamped at build time via ldflags.

cmd/terdut/main.go

@@ -1,5 +1,7 @@
 package main
 
+var version = "dev"
+
 import (
 	"context"
 	"log"
@@ -40,7 +42,7 @@ func main() {
 	defer stop()
 
 	go func() {
-		log.Printf("listening on %s", cfg.Addr)
+		log.Printf("terdut-server %s listening on %s", version, cfg.Addr)
 		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
 			log.Fatalf("listen: %v", err)
 		}