[Feature] Battle Pass system — 6-week seasons, free and paid tiers

## Overview
6-week season Battle Pass with free and paid tiers. Subscription gives free access to the paid tier. Unique items per season, only earnable through the pass — never restocked.

## Scope
- Season structure: 6-week cycles
- Free tier: available to all players, limited rewards
- Paid tier: purchasable or included with subscription
- Season-exclusive cosmetics — flags, patterns, effects
- Progress tracked per player per season
- Season end: unclaimed rewards expire, pass closes permanently

## Acceptance criteria
- [ ] Season timer visible in UI
- [ ] Free and paid tier tracks clearly differentiated
- [ ] Progress visible on player profile / HUD
- [ ] Subscription holders automatically granted paid tier access
- [ ] Season cosmetics marked as season-exclusive and never restocked
- [ ] Previous seasons archived and visible but not purchasable

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