feat(stripe): reuse customers and save cards for off-session reuse

[Tarunkumar0601]

Ties each payment to a reusable Stripe Customer and saves the card for later (off-session) charging.

• get_or_create_customer(): reuses Customer.stripe_customer_id, recovers by metadata, else creates — caching the id back
• Saves the card (setup_future_usage="off_session") so it can be charged again without the customer present
• SetupIntent flow to save a card without charging
• Robust against partial failures (no duplicate Stripe customers per party)

Prerequisite for subscriptions / recurring billing. Depends on feat/stripe-payment-intents-checkout.

Backport Needed: V15 & V16

[mergify]

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[greptile-apps]

Confidence Score: 3/5

Safe to merge for synchronous card payments only; async payment methods (SEPA, ACH, bank transfers) are silently never settled, and subscription price sync is broken due to a missing database column.

The synchronous card payment path (both Hosted Checkout and Embedded Elements) is well-guarded and largely correct. However, the async payment settlement path redirects buyers to the success page without ever finalising the ERPNext document — a webhook handler that the code explicitly relies on does not exist in this PR. Separately, the subscription sync writes to a database column (product_price_id on Subscription Plan) that the custom-field installer never creates, so every sync invocation logs an error and no Stripe price ID is ever persisted.

payments/payment_gateways/doctype/stripe_settings/stripe_settings.py (missing webhook endpoint for async settlement), payments/payment_gateways/stripe_subscription_sync.py + payments/utils/utils.py (product_price_id column absent from Subscription Plan custom fields)

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