Capture billing address details in checkout intents and checkout session flow

[Copilot]

This updates the self-service billing flow to capture billing address details during checkout instead of relying solely on Stripe-side retrieval after purchase. Billing address data is now persisted on CheckoutIntent, validated at the API boundary, and exposed through the existing checkout/admin surfaces.

• Checkout intent schema

  • Added persisted billing address fields to CheckoutIntent:
    • billing_address_country
    • billing_address_line_1
    • billing_address_line_2
    • billing_address_city
    • billing_address_state
    • billing_address_postal_code
  • Added the corresponding migration with nullable defaults so existing rows remain valid.
  • Updated model PII annotations to reflect stored location data.

• API contract

  • Extended checkout-intent and create-checkout-session serializers to accept and return billing address fields.
  • Added cross-field validation so billing address submission is all-or-nothing for required fields, while still allowing line_2 to remain optional.
  • Passed billing address values through CheckoutIntent.create_intent(...) so both direct intent creation and checkout-session creation persist the same shape.

• Stripe checkout behavior

  • Updated Stripe checkout session creation to require billing address collection and allow Stripe to write back customer address/name updates.
  • This keeps the hosted checkout flow aligned with the new persisted billing-address model.

• Admin and BFF exposure

  • Exposed billing address fields in the CheckoutIntent admin detail view.
  • Added billing address fields to the checkout BFF serializer so downstream consumers can read the stored values from checkout-intent payloads.

• Coverage

  • Added focused tests for:
    • model persistence/update behavior
    • serializer/API validation
    • checkout-intent create/update flows
    • migration defaults for existing rows
    • admin field visibility
    • Stripe checkout session configuration

Example shape now accepted by checkout creation endpoints:

{
  "enterprise_slug": "acme",
  "enterprise_name": "Acme Corp",
  "quantity": 25,
  "billing_address_country": "US",
  "billing_address_line_1": "123 Main St",
  "billing_address_line_2": "Suite 400",
  "billing_address_city": "Boston",
  "billing_address_state": "MA",
  "billing_address_postal_code": "02110"
}