feat: add ListTiers to subscriptions SDK interface

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

subscriptions/subscriptions.go

@@ -11,6 +11,7 @@ import (
 type Subscriptions interface {
 	GetUserTierLevel(ctx context.Context, userID uuid.UUID) (*TierLevel, error)
 	GetTierBySlug(ctx context.Context, slug string) (*Tier, error)
+	ListTiers(ctx context.Context) ([]Tier, error)
 	ListActivePlans(ctx context.Context, tierID uuid.UUID) ([]Plan, error)
 }