feat: #3620831 Let poll authors choose when vote counts are shown

Poll results always show the total number of votes and the number of votes per choice. Sites with a small audience do not want visitors to see how few people voted, and a theme override cannot hide the numbers without also dropping the percentages.

Add two base fields to the poll entity: vote_count_display with the modes never, always and threshold, and vote_count_threshold for the minimum number of votes in threshold mode. PollInterface gets the matching getters and setters and isVoteCountDisplayed(), which the results form asks before it adds the counts. When counts are hidden the poll_results template receives NULL for votes and the poll_meter labels carry the percentage only.

The default is always, so existing polls keep their output. The update hook installs the fields with that initial value. Percentages stay visible in every mode.

Closes #3620831

AI-Generated: Yes (Claude Code drafted the implementation and the functional test; reviewed and tested by kasperg)