[Feature] Subscriber Discord roles — Stripe → Discord tier sync

Auto-assign Discord roles based on subscription tier, as a subscriber perk.

### Scope
- [ ] Map subscription tiers → Discord roles in the OpenFront server.
- [ ] Auto-assign on purchase, auto-remove on lapse (Stripe webhook → Discord bot).
- [ ] Account linking (game/Stripe identity ↔ Discord user).
- [ ] Backfill for existing subscribers.

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

Parent: #4027