fix(platform): fix Stripe customer creation on FREE downgrade and dialog display issues

- Guard cancel_stripe_subscription with stripe_customer_id check to prevent
  creating orphaned Stripe customers for users who never had a paid subscription
- Fix upgrade dialog showing raw tier key (PRO) instead of human-readable label (Pro)
- Fix misleading footer text that contradicted downgrade-to-FREE behaviour
  (downgrades to Free are scheduled at period end, not immediate)

autogpt_platform/backend/backend/data/credit.py

@@ -1353,7 +1353,15 @@ async def cancel_stripe_subscription(user_id: str) -> bool:
     Raises stripe.StripeError if any modification fails, so the caller can avoid
     updating the DB tier when Stripe is inconsistent.
     """
-    customer_id = await get_stripe_customer_id(user_id)
+    # Guard: only proceed if the user already has a Stripe customer ID.  Calling
+    # get_stripe_customer_id for a user who has never had a paid subscription would
+    # create an orphaned, potentially-billable Stripe Customer object — we avoid that
+    # by returning False early so the caller can downgrade the DB tier directly.
+    user = await get_user_by_id(user_id)
+    if not user.stripe_customer_id:
+        return False
+
+    customer_id = user.stripe_customer_id
     try:
         cancelled_count = await _cancel_customer_subscriptions(
             customer_id, at_period_end=True

autogpt_platform/frontend/src/app/(platform)/profile/(user)/credits/components/SubscriptionTierSection/SubscriptionTierSection.tsx

@@ -196,8 +196,9 @@ export function SubscriptionTierSection() {
 
       {currentTier !== "FREE" && isPaymentEnabled && (
         <p className="text-sm text-neutral-500">
-          Your subscription is managed through Stripe. Changes take effect
-          immediately.
+          Your subscription is managed through Stripe. Upgrades and paid-tier
+          changes take effect immediately; downgrades to Free are scheduled for
+          the end of the current billing period.
         </p>
       )}
 
@@ -249,7 +250,9 @@ export function SubscriptionTierSection() {
               subscription.proration_credit_cents > 0 &&
               `Your unused ${currentTier.charAt(0) + currentTier.slice(1).toLowerCase()} subscription ($${(subscription.proration_credit_cents / 100).toFixed(2)}) will be added to your account balance. `}
             You will be redirected to Stripe to complete your upgrade to{" "}
-            {pendingUpgradeTier}.
+            {TIERS.find((t) => t.key === pendingUpgradeTier)?.label ??
+              pendingUpgradeTier}
+            .
           </p>
           <Dialog.Footer>
             <Button