[Feature] Vote for next map — subscriber-weighted map voting

Let subscribers influence the upcoming map selection, as a subscriber perk.

### Scope
- [ ] Voting surface (lobby or rotation) for upcoming map selection.
- [ ] Subscriber weighting / eligibility rules.
- [ ] Anti-abuse (dedupe, rate limit).
- [ ] Decide whether free players see results or participate at lower weight.

### Notes
- Perk definition should be confirmed in #4031 (subscription tier packages).

Parent: #4027