Fix the unnamed cron task that intermittently crashed pages, and add a CI debug workflow

[bonelifer]

Summary

Fixes the intermittent functional test failures, and a crash that can hit live boards.

• The bug: the restriction expiry cron task was registered without set_name, so its name was empty. On every page phpBB shuffles its cron tasks and triggers the first ready one from the page footer. Whenever it picked this one, building the cron URL threw
  InvalidParameterException: Parameter "cron_type" for route "phpbb_cron_run" must match "[^/]++" ("" given)
  and the page died with an empty HTTP 500. The shuffle is why it was intermittent, and why the failures moved between databases and test steps.
• The fix: config/services.yml now calls set_name with cron.task.phpbbmodders.banhammer.restriction_expiry, the same way every phpBB core cron task is registered.
• Adds .github/workflows/functional-debug.yml, which found the cause:
  • it mirrors the test framework's SQLite/MSSQL job, turns on PHP error logging in php-fpm (phpBB's CI setup leaves it off), runs the functional tests 10 times, and uploads the PHP, php-fpm and nginx logs
  • it runs only by hand (Actions → Functional test debug → Run workflow) or when the file changes in a pull request, so normal CI is unaffected

Test plan

• Before the fix, the debug workflow reproduced the failure in 2 of 5 runs on MSSQL 2022. Its PHP error log showed the exception above for both.
• Claude reproduced it on a phpBB 3.3.19 board: with the old services.yml the task has an empty name, building its cron URL throws the same exception, and phpBB can't find the task by name. With the fix, the task is named, its URL builds, and phpBB finds it.
• This PR's CI runs the debug workflow 10 times on MSSQL 2022, plus the normal tests.

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Investigated and written by Claude on behalf of William Jacoby (bonelifer).