fix: defer expiry reminders for trial and bonus subscriptions

Trial and registration/referral-bonus subscriptions usually last only a
few days, so the multi-day ending-soon reminders fired almost the moment
they were granted and needlessly alarmed newcomers.

Track this with a new subscription flag (suppress_early_expiry_notifications,
migration 0035): trial activation, referral welcome bonus and inviter bonus
grants set it, while a real paid purchase clears it on upsert. While set, the
notification worker skips the day-before stages but still sends the
hours-before reminder and the expiry/after-expiry notices, so users are
warned shortly before access ends. Once they pay for a full subscription the
complete reminder spectrum resumes.

backend/bot/services/subscription_service_impl/trial.py

@@ -63,6 +63,8 @@ class TrialSubscriptionMixin:
             "traffic_limit_bytes": self.settings.trial_traffic_limit_bytes,
             "auto_renew_enabled": False,
             "provider": "trial",
+            # Short trial: only warn a few hours before it ends, not days ahead.
+            "suppress_early_expiry_notifications": True,
         }
         try:
             await subscription_dal.upsert_subscription(session, trial_sub_data)