## Summary
Adds a 15-day trial to the monthly and yearly plans. Configurable per
plan (or disabled) via config.
## Changes
- `config/subscriptions.php` — new `trial_days` key per plan (defaults:
monthly=15, yearly=15). Env overrides: `STRIPE_PRO_MONTHLY_TRIAL_DAYS`,
`STRIPE_PRO_YEARLY_TRIAL_DAYS`. Set to `0` to disable.
- `SubscriptionController::checkout` — applies `trialDays()` on the
Cashier subscription builder when `trial_days > 0`.
- Tests — assert `trial_days` surfaced in pricing props; assert
`trialDays(15)` applied on checkout; assert skipped when `0`.
## Notes
Stripe Checkout enforces a **minimum 2-day trial**. Values of `1` will
fail at Stripe. `0` disables cleanly.
## Test plan
```
php artisan test --compact tests/Feature/SubscriptionTest.php
```
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