restore $PORT support, remove explicit ports mapping

The MCP agent assigns its own host port and sets $PORT. The app must
listen on $PORT for mc-proxy to reach it. Explicit Podman port mappings
in the service definition are ignored by the agent.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

main.go

@@ -6,6 +6,7 @@ import (
 	"log"
 	"net"
 	"net/http"
+	"os"
 
 	"git.wntrmute.dev/kyle/goutils/config"
 	"git.wntrmute.dev/kyle/goutils/die"
@@ -29,10 +30,15 @@ func main() {
 	srv := &server{db: db}
 	http.Handle("/", srv)
 
-	addr := net.JoinHostPort(
-		config.Get("HTTP_ADDR"),
-		config.GetDefault("HTTP_PORT", "8000"),
-	)
+	var addr string
+	if port := os.Getenv("PORT"); port != "" {
+		addr = ":" + port
+	} else {
+		addr = net.JoinHostPort(
+			config.Get("HTTP_ADDR"),
+			config.GetDefault("HTTP_PORT", "8000"),
+		)
+	}
 	log.Print("listening on ", addr)
 	log.Fatal(http.ListenAndServe(addr, nil))
 }