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-compose.dev.yml

@@ -0,0 +1,35 @@
+services:
+  postgres:
+    image: postgres:16-alpine
+    environment:
+      POSTGRES_USER: gearbox
+      POSTGRES_PASSWORD: gearbox
+      POSTGRES_DB: gearbox
+    ports:
+      - "5432:5432"
+    volumes:
+      - pgdata-dev:/var/lib/postgresql/data
+      - ./docker/init-logto-db.sql:/docker-entrypoint-initdb.d/init-logto-db.sql
+    healthcheck:
+      test: ["CMD-SHELL", "pg_isready -U gearbox"]
+      interval: 5s
+      timeout: 3s
+      retries: 5
+
+  logto:
+    image: svhd/logto:latest
+    depends_on:
+      postgres:
+        condition: service_healthy
+    entrypoint: ["sh", "-c", "npm run cli db seed -- --swe && npm start"]
+    ports:
+      - "3001:3001"
+      - "3002:3002"
+    environment:
+      TRUST_PROXY_HEADER: "1"
+      DB_URL: postgres://gearbox:gearbox@postgres:5432/logto
+      ENDPOINT: ${LOGTO_ENDPOINT:-http://localhost:3001}
+      ADMIN_ENDPOINT: ${LOGTO_ADMIN_ENDPOINT:-http://localhost:3002}
+
+volumes:
+  pgdata-dev: