Add postgres service (todo §3); pin postgres:18.4-alpine3.23, one DB per Kratos/Keto/Hydra via init.sql

ory/postgres/init/init.sql

@@ -0,0 +1,6 @@
+-- Runs once on first boot (docker-entrypoint-initdb.d), as the POSTGRES_USER.
+-- One database per Ory service: each owns its schema and runs its own migrations,
+-- so they never collide. The web app never connects here (stateless — see README).
+CREATE DATABASE kratos;
+CREATE DATABASE keto;
+CREATE DATABASE hydra;