[New Feature] Native polls (single/multi/ranked, time-limited)

What: Polls embedded in forum posts. Single-choice, multi-choice, ranked. Time-limited with auto-close. Member-results-visible toggle (hide results until vote OR until close). Live results.

Who benefits: Members (faster decisions in threads); admins (lightweight feedback tool).

Why: Circle ships polls. Discourse ships polls. We do not. Members work around with markdown hacks. Native polls are a 1-week ship that closes a basic gap.

Dependencies: None.

Recommended order: Sprint 2 (week 3-4). Stand-alone.

Complexity: 1 week. 1 model + 1 component + composer integration.

Priority: high