feat: Add /v1/capacity endpoint, 429 handler, and optional admission gate

[prein]

Depends on docling-project/docling-jobkit#126 (SystemCapacity + get_capacity()).

See #581 for full problem descriptio

Summary

• GET /v1/capacity - returns system-level capacity snapshot (queue depth, active jobs, worker count); cached with configurable TTL (default 2s) to avoid per-request Redis scans
• QueueLimitExceededError → 429 - fixes unhandled 500 when Ray queue limit rejection is enabled; returns capacity snapshot in response body
• Optional admission gate - new admission_max_queue_size setting; when set, rejects submissions with 429 before enqueue() if queue is full

Issue resolved by this Pull Request:
Resolves #581

[github-actions]

✅ DCO Check Passed

Thanks @prein, all your commits are properly signed off. 🎉

[mergify]

Merge Protections

🟢 Merge protection satisfied — ready to merge.

Show 1 satisfied protection

🟢 Enforce conventional commit

Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?(!)?:

[prein]

We are running it in our preprod environment and it works as expected. It would be amazing if you could find time to review it and hopefully merge and release so we don't have to maintain it in a fork.