[Feature] NPS question — post-game survey after match

## Overview
Show an NPS (Net Promoter Score) prompt in the post-game screen rotation. "How likely are you to recommend OpenFront to others?" Scale 1–10. Deduplicated per user.

## Scope
- NPS prompt shown in post-game screen rotation
- 1–10 scale
- Response stored against user if logged in, anonymously if not
- Deduplication: one response per user per rolling 90 days
- Results exportable for analysis

## Acceptance criteria
- [ ] NPS prompt appears in post-game rotation
- [ ] 1–10 scale renders correctly on mobile and desktop
- [ ] Logged-in user responses deduplicated (90-day cooldown)
- [ ] Anonymous responses accepted
- [ ] Results stored and exportable