✨(backend) add final_reference field to Invoice model

[jonathanreveille]

Purpose

The final reference field will be used when we generate on demand a final invoice. This may happen when orders are completed and all their installments are paid. This means the balance should be at 0. The final reference will be unique per organization. We also allow to generate the final invoice when a batch order uses the card payment.

🥚 Note : Not to be merged yet in main. This branch is the first PR for the implementation of this issue #1247

Proposal

• add new field final_reference in Invoice model
• add property and method to generate a unique reference per organization.

[Copilot]

Pull request overview

Adds support for generating and storing a “final invoice reference” on Invoice, intended for uniquely identifying finalized invoices per organization.

Changes:

• Add nullable unique final_reference field to Invoice (with a migration).
• Add eligibility logic (can_generate_final_reference) and reference generation/update methods on Invoice.
• Add tests covering final reference generation/uniqueness and eligibility behavior; document change in changelog.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 6 comments.

File	Description
src/backend/joanie/payment/models.py	Adds final_reference field and generation/eligibility logic on Invoice.
src/backend/joanie/payment/migrations/0013_invoice_final_reference.py	Creates DB column for final_reference.
src/backend/joanie/tests/payment/test_models_invoice.py	Adds tests for final reference uniqueness and eligibility rules.
CHANGELOG.md	Notes the addition of final_reference on invoices.

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