fix: complete mobile payment workflow end-to-end

[bllr777]

Summary

Closes the loop on mobile payments — Stripe now returns a hosted checkout URL, invoices are auto-created when missing, and the webhook correctly marks invoices paid after the customer pays.

Changes

Invoice auto-resolution (\EnsureInvoiceForPaymentAsync)

When a mobile worker taps Collect Payment, the API runs this waterfall:

1. Existing invoice for the job → use \BalanceDue\
2. Accepted estimate on the job → auto-create invoice from estimate line items (Option A)
3. No estimate + amount provided → create quick draft invoice with single line item (Option B)

Stripe checkout URL fix

• \StripePaymentProcessor: added \CreateOneTimeCheckoutSessionAsync\ using \Stripe.Checkout.Session\ (mode: payment) — returns a hosted redirect URL instead of a \clientSecret\
• \IPaymentProcessor\ / \SquarePaymentProcessor: interface + Square implementation added (Square already used hosted checkout)
• \PaymentController.Checkout: routes one-time payments through the new method; accepts \jobId\ to trigger invoice auto-resolution

Stripe webhook fix

\HandleCheckoutSessionAsync\ previously returned immediately for any session without a \SubscriptionId\ (i.e. every one-time payment). Now:

• Reads \invoiceId\ from session metadata
• Calls \MarkPaidAsync\
• Fires client + org payment notifications
• Marks onboarding \ReceivePayment\ step complete

Assignment BalanceDue surfacing

• \AssignmentDto: added \BalanceDue\ and \InvoiceId\
• \AssignmentService: pre-fetches latest invoice per job in \GetAssignmentsAsync\

Related

Pair PR in JobFlow-Mobile: fix: wire mobile payment flow end-to-end