Design offer and offer detail models

[FriggemannMichael]

Objective:
Create the data model for offers and the three required offer detail variants.

Acceptance criteria:

• Each offer belongs to a business user.
• Each offer has exactly basic, standard, and premium details.
• Offer details store title, revisions, delivery time, price, features, and offer_type.
• Model relationships use explicit related_name and correct on_delete behavior.
• String representations are useful in Django admin.

[FriggemannMichael]

TDD workflow for this project:

• Define the expected behavior before implementing the endpoint or model behavior.
• Write or outline the failing test first.
• Implement only enough code to make that test pass.
• Refactor after the test passes.
• Keep Django check, Ruff, and the relevant tests green from the local virtual environment.