feat(stripe): shared infrastructure for the stripe refactor

[Tarunkumar0601]

Foundation for the Stripe integration refactor — no behaviour change on its own.

• Adds stripe_utils.py: shared helpers (client, amount conversion, idempotency keys, customer/subscription resolution)
• Adds stripe_subscription_sync.py: syncs a Subscription Plan's price to Stripe
• Adds custom fields (Customer.stripe_customer_id, Subscription / Payment Entry Stripe ids) via patch + make_custom_fields()
• Adds new Stripe Settings fields (checkout mode, webhook secret, billing model, price-sync toggle)

Backport Needed: V15 & V16

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[greptile-apps]

Confidence Score: 4/5

Safe to merge as infrastructure with one outstanding crash path: if a Subscription Plan referenced in a Subscription Plan Detail row is later deleted, the bare tuple unpack in stripe_checkout.py line 51 raises TypeError on the checkout page.

The stripe_checkout.py change unpacks the result of frappe.db.get_value directly into billing_interval, billing_interval_count with no guard against a None return — a deleted or missing plan silently crashes the checkout page with a TypeError. This risk was flagged in a prior review thread but remains unaddressed.

payments/templates/pages/stripe_checkout.py — tuple unpack of frappe.db.get_value on line 51 needs a None guard before unpacking.

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[greptile-apps]

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[Sudharsanan11]

Make it a single if condition