[2.x] feat: flarum/audit support

[imorland]

Changes proposed in this pull request:

With flarum/audit enabled, polls logs:

• poll.created, poll.updated, poll.deleted, poll.published, poll.unpublished, with the poll's id and question;
• poll.voted, only when a vote changes something. For public polls it records the options added and removed. For private polls it doesn't, because their voters are hidden;
• poll_group.created, poll_group.updated, poll_group.deleted, with the group's id and name.

Entries for a poll in a discussion also carry discussion_id and post_id, so they appear when the log is filtered by that discussion. The audit browser labels are in en.yml under flarum-audit.lib.browser, as the bundled extensions do it. flarum/audit is a dev dependency and an optional dependency, and the extender is chained onto the existing Extend\Conditional.

The tests exposed two existing bugs, fixed in their own commits:

• PollVotesChanged::$unvotedOptionIds was always a list of nulls: it plucked option.id from the options themselves.
• DeletingPollGroup and DeletedPollGroup were never dispatched when a group was deleted through the API. Only the unused DeletePollGroupHandler dispatched them. PollGroupResource now does it in its delete hooks.

AuditTest covers each action, its payload, actor and IP, a resubmitted vote not being logged, and every action being registered with a label. flarum/audit's InteractsWithAuditLog helper isn't in its released package, so the few parts used are inlined. The label check reads en.yml directly, because extension locales aren't loaded in integration tests.

Reviewers should focus on:

Whether to record vote choices for public polls only, as here.

Confirmed

• Frontend changes: tested on a local Flarum installation. (No frontend changes.)
• Backend changes: tests are green (run composer test). PHPStan passes.