[Feature] Premium-only games — subscriber-exclusive match rotation

## Overview
Games that are only accessible to subscribers, running approximately every 10 minutes in the rotation. Exclusive gameplay perk for paid users.

## Scope
- Premium-only game mode visible but locked to non-subscribers
- Runs on a regular cadence in the game rotation
- Access gated behind any active subscription tier
- Non-subscribers see the game but receive a prompt to subscribe

## Acceptance criteria
- [ ] Premium games appear in rotation and are accessible to subscribers only
- [ ] Non-subscribers see a subscribe prompt on attempt to join
- [ ] Cadence configurable by admin
- [ ] Access correctly gated per subscription status

## Dependencies
- **Depends on:** Subscription tiers, OAuth