feat: add port option to HTTP API server for configurable listening port

main.ts

···
     )
     .description("Manage volumes")
     .command("serve", "Start the FreeBSD-Up HTTP API server")
+    .option("-p, --port <port:number>", "Port to listen on", { default: 8890 })
     .action(() => {
       serve();
     })

src/api/mod.ts

···
 import { logger } from "hono/logger";
 import { cors } from "hono/cors";
 import { bearerAuth } from "hono/bearer-auth";
+import { parseFlags } from "@cliffy/flags";
 
 export default function () {
   const token = Deno.env.get("FREEBSD_UP_API_TOKEN") ||
     crypto.randomUUID();
+  const { flags } = parseFlags(Deno.args);
 
   if (!Deno.env.get("FREEBSD_UP_API_TOKEN")) {
     console.log(`Using API token: ${token}`);
···
   app.route("/machines", machines);
   app.route("/volumes", volumes);
 
-  const port = Deno.env.get("FREEBSD_UP_PORT")
-    ? Number(Deno.env.get("FREEBSD_UP_PORT"))
-    : 8890;
+  const port = Number(
+    flags.port || flags.p ||
+      (Deno.env.get("FREEBSD_UP_PORT")
+        ? Number(Deno.env.get("FREEBSD_UP_PORT"))
+        : 8890),
+  );
 
   Deno.serve({ port }, app.fetch);
 }