feat: add configurable persistent state file path and env tests

src/config.js

@@ -10,8 +10,14 @@ function intFromEnv(name, fallback) {
   return Number.isInteger(parsed) ? parsed : fallback;
 }
 
+function strFromEnv(name, fallback) {
+  const raw = process.env[name];
+  return raw && String(raw).trim() ? String(raw).trim() : fallback;
+}
+
 const config = {
   port: intFromEnv("PORT", 3000),
+  stateFilePath: strFromEnv("STATE_FILE_PATH", "./data/state.json"),
   xWebhookSecret: process.env.X_WEBHOOK_SECRET || "dev-x-secret",
   polarWebhookSecret: process.env.POLAR_WEBHOOK_SECRET || "dev-polar-secret",
   credit: {