test: add local ECS runtime smoke test

[suguanYang]

Summary

• add an isolated Docker runtime smoke test for the current API and worker images
• run the complete Alembic migration graph against disposable PostgreSQL
• verify API and worker container health with disposable Redis
• document why LocalStack Community cannot validate ECS/Fargate orchestration

Safety

• calls no AWS API
• uses filesystem object storage and mock LLM responses
• removes its temporary containers and Docker network on exit
• supports alternate PostgreSQL/Redis images and Docker build proxy arguments

Verification

• ./deploy/ecs/docker-runtime-smoke.sh passed
• current API and worker images built successfully
• Alembic migrations completed
• API and worker runtime health checks passed
• temporary containers and network were confirmed absent afterward
• bash -n deploy/ecs/docker-runtime-smoke.sh passed
• python3 -m pytest -q deploy/ecs/test_render_task_definitions.py passed (4 tests)
• git diff --check passed