feat: add subscription reissue with 15-min cooldown

- Add revoke handler for classic and multi-tariff modes with 2-step
  confirmation dialog and TOCTOU-safe cooldown enforcement
- Add cabinet API endpoint POST /subscription/revoke with 429 + Retry-After
  for cooldown, IDOR protection via resolve_subscription
- Add last_revoke_at column to subscriptions (Alembic migration 0071)
- Add SUBSCRIPTION_REVOKE_ENABLED and COOLDOWN_SECONDS config settings
- Add revoke button to classic subscription settings keyboard and
  multi-tariff detail keyboard (gated by feature toggle)
- Add locale keys for revoke UI in all 5 languages (ru, en, ua, zh, fa)

app/database/models.py

@@ -1808,6 +1808,7 @@ class Subscription(Base):
     updated_at = Column(AwareDateTime(), default=func.now(), onupdate=func.now())
 
     last_webhook_update_at = Column(AwareDateTime(), nullable=True)
+    last_revoke_at = Column(AwareDateTime(), nullable=True)
 
     remnawave_short_uuid = Column(String(255), nullable=True)
     remnawave_uuid = Column(String(255), nullable=True)