feat(15-01): add Logto service to Docker Compose and create init script

- Add Logto OIDC provider to docker-compose.yml and docker-compose.dev.yml
- Create docker/init-logto-db.sql to initialize separate Logto database on Postgres
- Add OIDC env vars (issuer, client ID/secret, auth secret) to app service
- Document all required env vars in .env.example

docker/init-logto-db.sql

@@ -0,0 +1,2 @@
+-- Creates a separate database for Logto on the shared Postgres instance
+CREATE DATABASE logto;