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

[iiamlewis]

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 [Feature] Define the three subscription tier packages — product decision #4031 (subscription tier packages).

Parent: #4027