Fix enabling the extension on a fresh board

[bonelifer]

Summary

• Enabling the extension always failed on a fresh board with "You have requested a non-existent service phpbbmodders.documentation.permission_sync". The install migration asked the container for this extension's own service, but during the first enable the container doesn't include the extension's services yet (it only counts as enabled once its migrations finish).
• When the service isn't available, the migration now sets a phpbbmodders_documentation_sync_pending flag instead, and the event listener runs the permission sync on the next request. set_atomic() claims the flag, so the sync runs only once. Boards that already ran the migration are unaffected.

Test plan

Claude tested on a fresh phpBB 3.3.19 board (SQLite, PHP 8.4) with a small docs build in place:

• main: enabling fails with the error above.
• This branch:
  • enabling succeeds and leaves the sync pending
  • the first page load runs it and clears the flag
  • the language and section permissions are created
  • a guest can read the docs home and a section page
  • board pages and the ACP module load with no PHP errors

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