Add env/config loader (todo §0); validate at boot, wire port into server

src/server.ts

@@ -1,6 +1,7 @@
 import { createApp } from "./app.ts";
+import { loadConfig } from "./config.ts";
 
-const port = Number(process.env["PORT"] ?? 3000);
+const { port } = loadConfig(); // validates the env (incl. prod secrets) — fails loud at boot
 
 createApp().listen(port, () => {
   console.log(`Listening on http://localhost:${port}`);