perf(worker): cap Fargate concurrency at 10

[suguanYang]

Summary

• set WORKER_CONCURRENCY=10 explicitly in the staging Fargate task definition
• extend the ECS rendering contract so the concurrency target cannot drift with the application default

Rationale

The previous staging task omitted this environment value and inherited gevent concurrency 50. Under the prior pressure run, the single 0.5-vCPU worker saturated and lost health. This change establishes the owner-approved per-task target before the controlled capacity re-baseline.

Validation

• uv run pytest deploy/ecs/test_render_task_definitions.py — 6 passed
• uv run --group lint ruff check deploy/ecs/render_task_definitions.py deploy/ecs/test_render_task_definitions.py — passed
• full repository suite — 472 passed; 4 unrelated existing failures in summary-builder/page-memory tests

Supports Ontos-AI/knowhere-api-infra#22.