chore: add .env.example for local setup

Document PORT, Firebase credentials, FCM token data dir, and test-local
NODE_ENV so developers can copy the file to .env without guessing vars.

.env.example

@@ -0,0 +1,12 @@
+# HTTP port (default: 3000)
+PORT=3000
+
+# Firebase Admin: inline service account JSON (one line).
+# If unset, uses Application Default Credentials (e.g. GOOGLE_APPLICATION_CREDENTIALS).
+# FIREBASE_SERVICE_ACCOUNT_JSON={"type":"service_account",...}
+
+# Local persistence directory for registered FCM tokens (default: ./data)
+# FCM_TOKEN_DATA_DIR=./data
+
+# Set to "test-local" to bypass ethr JWT expiry verification in local dev only.
+# NODE_ENV=test-local