Comprehensive fixes to prevent customer ID mismatches:
1. **Checkout creation now passes customer_id** (lines 13669, 13064, 13503):
- All checkouts (subscription, topup, PAYG) now call ensureDodoCustomer() first
- Pass existing customer_id to checkout body to prevent Dodo from creating duplicates
- Added customerId to metadata for tracking
2. **Subscription confirmation validates customer consistency** (line 13843):
- Logs warning when checkout returns different customer_id than stored
- Tracks which customer_id was used in checkout metadata
- Prevents silent customer ID overwrites
3. **ensureDodoCustomer handles multiple customers per email** (line 6774):
- Logs warning when multiple customers found for same email
- Checks ALL customers for active subscriptions
- Selects customer with active subscriptions if multiple exist
- Returns first customer only if no active subscriptions found
4. **Added missing return statements** (lines 12417, 12448):
- Prevents double response errors after successful plan changes
This ensures that:
- New subscriptions use existing customers instead of creating duplicates
- Plan changes work correctly even with multiple subscriptions
- Customer ID mismatches are detected and logged
- The correct customer (one with active subscriptions) is always used
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