Add .env.example documenting required secrets

.env.example

@@ -0,0 +1,12 @@
+# Copy to .env and fill in. Never commit the real .env.
+
+# Long random string — used to sign admin login tokens.
+# Generate one with:  node -e "console.log(require('crypto').randomBytes(48).toString('hex'))"
+JWT_SECRET=
+
+# Used only by `npm run seed` to create the first admin account.
+ADMIN_USER=admin
+ADMIN_PASS=change-me-to-something-strong
+
+# Port the Node server listens on (nginx reverse-proxies to this).
+PORT=3000