Make the callback flow reusable for secondary payment models

[PetrDlouhy]

Depends on #PR1 — this branch contains that commit until it merges. Please review it after (or alongside) that one.

The problem

PAYMENT_MODEL names a single model, but projects commonly charge through several — subscriptions, marketplace orders, card verification — each with its own model and its own callback URL. There is currently no way to reuse process_data for the additional ones, so those projects copy the view into their codebase.

Copies drift, and the drift is invisible until it costs something. Concretely: a project of ours carries four such copies. When process_data learned to lock the payment row (#PR1), none of the copies did, and all four kept the race that had already demoted a captured payment to error in production. Any future fix here has the same problem.

The change

Split the view into the two steps it always was, and let it take the model:

def get_payment_or_404(token, payment_model=None):      # the locked fetch
def process_payment_data(payment, request, provider=None):  # the dispatch
def process_data(request, token, provider=None, payment_model=None):

process_data's behavior is unchanged when payment_model is not passed.

• A project whose extra model needs no custom logic routes the real view and inherits later fixes automatically:

  path("marketplace/process/<uuid:token>/", process_data,
       {"payment_model": MarketplacePayment}, name="process_marketplace_payment"),

• One that needs logic around the provider call composes the two helpers and still gets the locked fetch.

Both are documented under Using more than one payment model in docs/payment-model.rst.

Two deliberate choices:

• payment_model, not a queryset argument. A queryset parameter would let callers pass an unlocked or pre-filtered one and silently lose the serialization the fetch exists to provide.
• _default_manager instead of .objects. That is what get_object_or_404 uses when handed a model class, so adding the lock does not change which manager a project's payment model is read through. (It also types correctly — type[Model].objects does not.)

Tests

Written test-first; the new tests fail without this change. They cover the default and explicit model paths, provider resolution and its Http404, an explicitly passed provider, and the view with an explicit model. Full suite passes; ruff and mypy clean.