feat: add SESSION_SECRET_FILE and OAUTH_CLIENT_SECRET_FILE env vars

lib/read-file-env.ts

@@ -0,0 +1,13 @@
+import { readFileSync } from "fs";
+
+export function readFileEnv(path: string | undefined): string | null {
+	if (!path) {
+		return null;
+	}
+
+	try {
+		return readFileSync(path, "utf-8").trim();
+	} catch {
+		return null;
+	}
+}