improvement: docker infrastructure overhauled.

infrastructure/backend.staging.env.example

@@ -0,0 +1,12 @@
+# Copy to backend.staging.env — DATABASE_URL must match database.staging.env credentials.
+NODE_ENV=production
+PORT=3000
+
+DATABASE_URL=postgresql://postgres:changeme_staging_strong_password@postgres:5432/dyolink_db
+
+JWT_SECRET=replace_with_a_long_random_secret
+JWT_EXPIRES_IN=15m
+JWT_REFRESH_SECRET=another_long_random_secret_different_from_JWT_SECRET
+JWT_REFRESH_EXPIRES_IN=30d
+
+FRONTEND_URL=http://178.131.50.201:8088