fix(payment): pin promo-code checkout presentment to USD so the $269 minimum passes (#23299) (#23557)

## Summary

Disable Stripe Adaptive Pricing on checkout sessions created with a
resolved promotion code, pinning presentment to the price's native USD
currency.

Stripe evaluates a promotion code's `minimum_amount` restriction against
the checkout session's presentment currency. With Adaptive Pricing
enabled, a EUR customer's converted total (e.g. €249 for the $269
Unlimited yearly price) falls below the USD-authored $269 minimum on the
Unlimited yearly activation codes, and redemption fails with "code is
valid but order total is below the minimum amount" even though the code
is valid. With presentment pinned to USD, the minimum passes and the
100%-off first-year coupon applies for any customer.

Scope note: minimal durable fix by design. It does not add a coupon
fallback — the upgrade path is repo-contracted to apply promotion codes,
never raw coupons, to preserve Stripe-side restrictions and redemption
tracking — and it does not change trial handling (#23300 tracks that
separately).

Fixes #23299

## Failure-Class
Failure-Class: none

## Testing

- New `backend/tests/unit/test_payment_promo_adaptive_pricing.py`: promo
sessions set `adaptive_pricing={'enabled': False}`; promo-free sessions
keep Stripe's default; static contract scopes the pin to the promo
branch.
- Existing `backend/tests/unit/test_payment_promotion_codes.py` stays
green — its no-coupon contract on checkout/upgrade is untouched.
- Both suites run locally green on this head via the worktree venv;
backend CI re-runs them on this PR.

Author: Git-on-my-level

backend/tests/unit/test_payment_promo_adaptive_pricing.py

@@ -0,0 +1,73 @@
+"""Promotion-code checkouts pin presentment currency (issue #23299).
+
+Stripe evaluates a promotion code's ``minimum_amount`` restriction against the
+checkout session's presentment currency. With Adaptive Pricing enabled, the
+converted total for a foreign-currency customer (e.g. EUR 249 for the $269
+Unlimited yearly price) falls below the USD-authored minimum and a valid
+activation code is rejected with "order total is below the minimum amount".
+
+Sessions created with a resolved promotion code must disable Adaptive Pricing
+so the minimum is evaluated against the price's native USD amount. Sessions
+without a promotion code keep Stripe's Adaptive Pricing default.
+"""
+
+import os
+import sys
+from pathlib import Path
+from types import SimpleNamespace
+from unittest.mock import patch
+
+os.environ.setdefault("BASE_API_URL", "http://localhost:8080")
+
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..', '..'))
+
+import utils.stripe as stripe_utils  # noqa: E402
+
+STRIPE_UTILS_SOURCE = Path(__file__).resolve().parents[2] / "utils" / "stripe.py"
+
+
+def _captured_session_params(promotion_code_id):
+    captured = {}
+
+    def fake_session_create(**params):
+        captured.update(params)
+        return SimpleNamespace(url='https://checkout.stripe.com/c/pay/cs_test', id='cs_test')
+
+    with patch('utils.stripe.stripe.checkout.Session.create', side_effect=fake_session_create):
+        stripe_utils.create_subscription_checkout_session(
+            uid='uid_1',
+            price_id='price_unlimited_yearly',
+            idempotency_key='idem_1',
+            promotion_code_id=promotion_code_id,
+        )
+    return captured
+
+
+def test_promo_checkout_disables_adaptive_pricing():
+    """A promo-code session pins presentment to the price's USD currency."""
+    params = _captured_session_params(promotion_code_id='promo_123')
+
+    assert params['discounts'] == [{'promotion_code': 'promo_123'}]
+    assert params['adaptive_pricing'] == {'enabled': False}
+
+
+def test_promo_free_checkout_keeps_adaptive_pricing_default():
+    """Without a promo code the session must not set adaptive_pricing, so
+    Stripe's default (Adaptive Pricing where enabled) still applies."""
+    params = _captured_session_params(promotion_code_id=None)
+
+    assert params['allow_promotion_codes'] is True
+    assert 'adaptive_pricing' not in params
+    assert 'discounts' not in params
+
+
+def test_adaptive_pricing_pin_is_scoped_to_promo_sessions():
+    """Static contract: the pin lives inside the promotion_code_id branch, so a
+    refactor can never silently widen it to promo-free checkouts."""
+    source = STRIPE_UTILS_SOURCE.read_text(encoding="utf-8")
+
+    branch_start = source.index("if promotion_code_id:")
+    branch_end = source.index("session_params['allow_promotion_codes'] = True", branch_start)
+    promo_branch = source[branch_start:branch_end]
+
+    assert "session_params['adaptive_pricing'] = {'enabled': False}" in promo_branch

backend/utils/stripe.py

@@ -84,6 +84,11 @@ def create_subscription_checkout_session(
 
         if promotion_code_id:
             session_params['discounts'] = [{'promotion_code': promotion_code_id}]
+            # Pin presentment to the price's currency. Stripe evaluates a promotion
+            # code's minimum_amount restriction against the checkout's presentment
+            # currency, so Adaptive Pricing's converted total (e.g. EUR) can fall
+            # below a USD-authored minimum and reject a valid code (issue #23299).
+            session_params['adaptive_pricing'] = {'enabled': False}
         else:
             session_params['allow_promotion_codes'] = True