Supabase

Supabase gives each project a full Postgres database plus the pieces most apps need around it: authentication, file storage, realtime subscriptions, edge functions and auto-generated APIs. You can query from client libraries for JavaScript, Flutter, Swift and others, or connect to Postgres directly, and Row Level Security keeps access rules in the database itself.

Supabase is open source, so it can be self-hosted, or used as a managed cloud service with a free tier and usage-based paid plans. It's often compared with Firebase; the difference is that your data lives in standard Postgres.