Closes PR #200: Reconcile Supabase with Stripe in createOrRetrieveCustomer

[chriscarrollsmith]

Fixes and closes #200, which fails if the customer already exists in both Supabase and Stripe.

The code in that PR looks a Stripe record and then tries to upsert a duplicate record to Supabase without checking if a Supabase record exists first, which raises an error if a Supabase record exists.

I refactored the code so it should perform reconciliation in all edge cases.

• If both Supabase and Stripe records exist but the IDs don't match, it updates the Supabase record.
• If a record exists in Stripe but not in Supabase, it inserts a new Supabase record with the existing Stripe ID.
• If a record exists in Supabase but not in Stripe, it creates a new Stripe record and updates the Supabase record with the new Stripe ID.
• If no record exists in Stripe, it creates a new customer in both Stripe and Supabase.

This is a fairly aggressive reconciliation process, but it should be robust and should help correct human errors caused by manually deleting database entries.

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