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/db/migrator.py

@@ -1143,6 +1143,18 @@ def _migration_0034_add_legacy_import_compatibility(connection: Connection) -> N
     )
 
 
+def _migration_0035_add_subscription_promo_expiry_flag(connection: Connection) -> None:
+    inspector = inspect(connection)
+    columns: Set[str] = {col["name"] for col in inspector.get_columns("subscriptions")}
+    if "suppress_early_expiry_notifications" not in columns:
+        connection.execute(
+            text(
+                "ALTER TABLE subscriptions ADD COLUMN suppress_early_expiry_notifications "
+                "BOOLEAN NOT NULL DEFAULT FALSE"
+            )
+        )
+
+
 MIGRATIONS: List[Migration] = [
     Migration(
         id="0001_add_channel_subscription_fields",
@@ -1325,6 +1337,11 @@ MIGRATIONS: List[Migration] = [
         description="Store legacy import mappings and referral codes for source-bot migrations",
         upgrade=_migration_0034_add_legacy_import_compatibility,
     ),
+    Migration(
+        id="0035_add_subscription_promo_expiry_flag",
+        description="Suppress multi-day expiry reminders for trial and bonus subscriptions",
+        upgrade=_migration_0035_add_subscription_promo_expiry_flag,
+    ),
 ]