docs: correct arq lifecycle description in README

[lesnik512]

Summary

The README usage paragraph said setup_di wires four arq hooks but listed three, and said the per-job child is open only while @Inject bodies run.

Evidence:

• modern_di_arq/main.py setup_di sets on_job_end via _wrap_job_end, which closes the child if it is still open (fallback).
• _wrap_job_start builds the child in on_job_start (open from construction, per the setup_di docstring); inject's wrapper refcounts and the last call to exit runs close_async().

Changes

• Name on_job_end as the fourth hook and its fallback role.
• State that the child opens in on_job_start and the last @inject call closes it.
• Split the ~70-word sentence; remove dashes and the bold "must".
• Footer: "with IoC container" -> "with an IoC container", and the org-profile line no longer uses a dash.

Checklist

• Lint and format pass (ruff)
• Type check passes (ty)
• Tests pass and new behavior is covered
• Build succeeds (uv build) if packaging or build config changed
• Repo metadata stays consistent across the three surfaces (GitHub description, pyproject description, profile blurb) if this touches packaging

Docs-only change. just lint-ci and just test pass; no mkdocs.yml in this repo.