* fix(job_key): hash the URL alone so a re-listed non-Latin title keeps one key
* docs(changelog): record the job_key URL-identity fix
Adds the [Unreleased] / Fixed entry for #501, naming tools/job_key.py and
tests/test_job_key_url_identity.py.
The entry states the migration consequence in bold: an entry written before
this fix, for a non-Latin title on a portal without a numeric id, re-keys once
on the next scrape. It also names where those entries surface,
keys_not_matching_current_rule, and that --audit never rewrites them.
Verified that claim rather than asserting it. A seen_jobs.json holding one
entry under its pre-fix key for a Cyrillic-titled freehire URL reports
keys_not_matching_current_rule: 1 under the fixed audit. The same audit over a
real 884-entry seen_jobs.json reports 0, because those keys are the
company_title form that is_canonical() excludes from the drift check.
