fix(deploy): keep production API at 2 GiB

[suguanYang]

Summary

• parameterize API ECS CPU and memory in the shared task-definition renderer
• keep staging API at 256 CPU / 1024 MiB
• set production API releases to 512 CPU / 2048 MiB (2 GiB)
• document the capacity contract and cover the renderer with contract tests

This prevents a future production release from reverting the API task to the previous 1 GiB default.

Verification

• pytest -q deploy/ecs/test_render_task_definitions.py (8 passed)
• git diff --check

Deployment note

The current production task definition remains unchanged until this PR is merged and a normal production release is run.