Fix PORT env var overwriting the 8080 fallback

The old code set port='8080' then unconditionally overwrote it with
os.Getenv("PORT"), which returns "" when unset, causing the server
to bind to ':' instead of ':8080'.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cmd/web/main.go

@@ -16,14 +16,13 @@ import (
 var appKey []byte
 
 func main() {
-	var port string
+	port := "8080"
 
-	err := godotenv.Load()
-	if err != nil {
-		port = "8080"
+	godotenv.Load()
+
+	if p := os.Getenv("PORT"); p != "" {
+		port = p
 	}
-
-	port = os.Getenv("PORT")
 	appKey = []byte(os.Getenv("APP_KEY"))
 	if len(appKey) == 0 {
 		log.Fatal("unable to load APP_KEY")