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

.env.example

@@ -0,0 +1,15 @@
+# PostgreSQL
+POSTGRES_PASSWORD=changeme
+
+# Logto OIDC (get from Logto Admin Console at http://localhost:3002)
+LOGTO_ENDPOINT=http://localhost:3001
+LOGTO_ADMIN_ENDPOINT=http://localhost:3002
+LOGTO_CLIENT_ID=your-app-client-id
+LOGTO_CLIENT_SECRET=your-app-client-secret
+OIDC_AUTH_SECRET=generate-a-random-32-char-string-here
+
+# Derived (set in docker-compose.yml, not needed here):
+# OIDC_ISSUER=${LOGTO_ENDPOINT}/oidc
+
+# GearBox
+GEARBOX_URL=http://localhost:3000